clean up stdio_impl.h

this header evolved to facilitate the extremely lazy practice of
omitting explicit includes of the necessary headers in individual
stdio source files; not only was this sloppy, but it also increased
build time.

now, stdio_impl.h is only including the headers it needs for its own
use; any further headers needed by source files are included directly
where needed.
clean up sloppy nested inclusion from pthread_impl.h

this mirrors the stdio_impl.h cleanup. one header which is not
strictly needed, errno.h, is left in pthread_impl.h, because since
pthread functions return their error codes rather than using errno,
nearly every single pthread function needs the errno constants.

in a few places, rather than bringing in string.h to use memset, the
memset was replaced by direct assignment. this seems to generate much
better code anyway, and makes many functions which were previously
non-leaf functions into leaf functions (possibly eliminating a great
deal of bloat on some platforms where non-leaf functions require ugly
prologue and/or epilogue).
fix "configure --prefix=" and improve path/arg handling in configure

previously, empty string was treated as "use default". this is
apparently not compatible with standard configure semantics where an
empty prefix puts everything under /. the new logic should be a lot
cleaner and not suffer from such issues.
change ldso path file logic to replace rather than add to search path

this change was originally intended just to avoid repeated attempts to
open a nonexistant /etc/ls-musl-$(ARCH).path file, but I realized it
also prevents the default paths from being searched when such a path
file exists. despite the potential to break existing usage, I believe
the new behavior is the right behavior, and it's better to fix it
sooner rather than later. with the old behavior, it was impossible to
inhibit search of default paths which might contain musl-incompatible
libs (or even libs from a different cpu arch, on multi-arch machines).
support ldso path files without final newline
always add memory streams to stdio open file list

per interpretation for austin group issue #626, fflush(0) and exit()
must block waiting for a lock if another thread has locked a memory
stream with flockfile. this adds some otherwise-unnecessary
synchronization cost to use of memory streams, but there was already a
synchronization cost calling malloc anyway.

previously the stream was only added to the open file list in
single-threaded programs, so that upon subsequent call to
pthread_create, locking could be turned on for the stream.
fix clobber of edx in i386 vsyscall asm

this function does not obey the normal calling convention; like a
syscall instruction, it's expected not to clobber any registers except
the return value. clobbering edx could break callers that were reusing
the value cached in edx after the syscall returns.
add support for thread scheduling (POSIX TPS option)

linux's sched_* syscalls actually implement the TPS (thread
scheduling) functionality, not the PS (process scheduling)
functionality which the sched_* functions are supposed to have.
omitting support for the PS option (and having the sched_* interfaces
fail with ENOSYS rather than omitting them, since some broken software
assumes they exist) seems to be the only conforming way to do this on
linux.
report support of TPS option in unistd.h and sysconf

also update another newish feature in sysconf, stackaddr
debloat src/thread tree but putting lots of junk in one file

POSIX includes mostly-useless attribute-get functions for each
attribute-set function, presumably out of some object-oriented
dogmatism. the get functions are not useful with the simple idiomatic
usage of attributes. there are of course possible valid uses of them
(like writing wrappers for pthread init functions that perform special
actions on the presence of certain attributes), but considering how
tiny these functions are anyway, little is lost by putting them all in
one file, and some build-time cost and archive-file-size benefits are
achieved.
math: fix long double constants in exp10l.c
math: fix exception behaviour of expm1l.c on inf and nan
math: raise invalid flag in ilogb*.c on +-0, +-inf and nan
math: raise exception flags in log1pl.c on <= -1 arguments
math: raise flags in log2l.c on <= 0 arguments, and fix volatile
math: fix logb*.c exceptions now that ilogb raises invalid
math: raise flags in logl.c on <= 0 arguments
complex: add C11 CMPLX macros and replace cpack with them
math: fix nextafter and nexttoward on maxdbl and maxflt input

old code (return x+x;) returns correct value and raises correct
flags only if the result is stored as double (or float)
math: fix scalbn and scalbnf on overflow/underflow

old code was correct only if the result was stored (without the
excess precision) or musl was compiled with -ffloat-store.
(see note 160 in n1570.pdf section 6.8.6.4)