dynamic linker: permit error returns from arch-specific reloc function

the immediate motivation is supporting TLSDESC relocations which
require allocation and thus may fail (unless we pre-allocate), but
this mechanism should also be used for throwing an error on
unsupported or invalid relocation types, and perhaps in certain cases,
for reporting when a relocation is not satisfiable.
multiple fixes to sh (superh) dynamic linker relocations

the following issues are fixed:

- R_SH_REL32 was adding the load address of the module being relocated
  to the result. this seems to have been a mistake in the original
  port, since it does not match other dynamic linker implementations
  and since adding a difference between two addresses (the symbol
  value and the relocation address) to a load address does not make
  sense.

- R_SH_TLS_DTPMOD32 was wrongly accepting an inline addend (i.e. using
  += rather than = on *reloc_addr) which makes no sense; addition is
  not an operation that's defined on module ids.

- R_SH_TLS_DTPOFF32 and R_SH_TLS_TPOFF32 were wrongly using inline
  addends rather than the RELA-provided addends.

in addition, handling of R_SH_GLOB_DAT, R_SH_JMP_SLOT, and R_SH_DIR32
are merged to all honor the addend. the first two should not need it
for correct usage generated by toolchains, but other dynamic linkers
allow addends here, and it simplifies the code anyway.

these issues were spotted while reviewing the code for the purpose of
refactoring this part of the dynamic linker. no testing was performed.
fix powerpc dynamic linker thread-pointer-relative relocations

processing of R_PPC_TPREL32 was ignoring the addend provided by the
RELA-style relocation and instead using the inline value as the
addend. this presumably broke dynamic-linked access to initial TLS in
cases where the addend was nonzero.
refactor to remove arch-specific relocation code from dynamic linker

this was one of the main instances of ugly code duplication: all archs
use basically the same types of relocations, but roughly equivalent
logic was duplicated for each arch to account for the different naming
and numbering of relocation types and variation in whether REL or RELA
records are used.

as an added bonus, both REL and RELA are now supported on all archs,
regardless of which is used by the standard toolchain.
reduce code duplication in dynamic linker error paths

eventually this should help making dlerror thread-safe too.
fix incorrect comparison loop condition in memmem

the logic for this loop was copied from null-terminated-string logic
in strstr without properly adapting it to work with explicit lengths.

presumably this error could result in false negatives (wrongly
comparing past the end of the needle/haystack), false positives
(stopping comparison early when the needle contains null bytes), and
crashes (from runaway reads past the end of mapped memory).
add arch-generic support for tlsdesc relocations to dynamic linker

this code is non-functional without further changes to link up the
arch-specific reloc types for tlsdesc and add asm implementations of
__tlsdesc_static and __tlsdesc_dynamic.
change dynamic TLS installation strategy to optimize access

previously, accesses to dynamic TLS had to check two conditions before
being able to use a dtv slot: (1) that the module index was within the
bounds of the current dtv size, and (2) that the dynamic tls for the
requested module index was already installed in the dtv.

this commit changes the installation strategy so that, whenever an
attempt is made to access dynamic TLS that's not yet installed in the
dtv, the dynamic TLS for all lower-index modules is also installed.
thus it provides a new invariant: if a given module index is within
the bounds of the current dtv size, we automatically know that its TLS
is installed and directly available. the requirement that the second
condition (above) be checked is eliminated.
optimize i386 ___tls_get_addr asm
add tlsdesc support for i386
separate __tls_get_addr implementation from dynamic linker/init_tls

such separation serves multiple purposes:

- by having the common path for __tls_get_addr alone in its own
  function with a tail call to the slow case, code generation is
  greatly improved.

- by having __tls_get_addr in it own file, it can be replaced on a
  per-arch basis as needed, for optimization or ABI-specific purposes.

- by removing __tls_get_addr from __init_tls.c, a few bytes of code
  are shaved off of static binaries (which are unlikely to use this
  function unless the linker messed up).
add tlsdesc support for x86_64
implement sendmmsg and recvmmsg

these are not pure syscall wrappers because they have to work around
kernel API bugs on 64-bit archs. the workarounds could probably be
made somewhat more efficient, but at the cost of more complexity. this
may be revisited later.
rename dynamic linker entry point from _start to _dlstart

the main motivation for this change is to aid in debugging. since the
main program's entry point is also named _start, it was difficult to
set breakpoints or quickly identify which _start execution stopped in.
fix sendmmsg emulation return value for zero-length vector

this case is not even documented, but the kernel returns 0 here and it
makes sense to be consistent.
fix gethostby*_r result pointer value on error

according to the documentation in the man pages, the GNU extension
functions gethostbyaddr_r, gethostbyname_r and gethostbyname2_r are
guaranteed to set the result pointer to NULL in case of error or no
result.
remove optimization-inhibiting behavior from configure's --enable-debug

this behavior turned out to be counter-intuitive to users and in any
case it's unnecessary. optimization can be disabled explicitly using
the --disable-optimize option, or both can be achieved without any
enable/disable options by passing CFLAGS="-O0 -g".
remove hack in syslog.h that resulted in aliasing violations

this issue affected the prioritynames and facilitynames arrays which
are only provided when requested (usually by syslogd implementations)
and which are presently defined as compound literals. the aliasing
violation seems to have been introduced as a workaround for bad
behavior by gcc's -Wwrite-strings option, but it caused compilers to
completely optimize out the contents of prioritynames and
facilitynames since, under many usage cases, the aliasing rules prove
that the contents are never accessed.
implement result address sorting in the resolver (getaddrinfo, etc.)
implement fmtmsg function

contributed by Isaac Dunham. this seems to be the last interface that
was missing for complete POSIX 2008 base + XSI coverage.