disable SO_REUSEPORT in sys/socket.h

although a number is reserved for it, this option is not implemented
on Linux and does not work. defining it causes some applications to
use it, and subsequently break due to its failure.
fix powerpc termios.h and ioctl.h
fix ppc stat structure definition
dynamic linking support for powerpc

incomplete but at least partly working. requires all files to be
compiled in the new "secure" plt model, not the old one that put plt
code in the data segment. TLS is untested but may work. invoking the
dynamic linker explicitly to load a program does not yet handle argv
correctly.
math: exp.c clean up

overflow and underflow was incorrect when the result was not stored.
an optimization for the 0.5*ln2 < |x| < 1.5*ln2 domain was removed.
did various cleanups around static constants and made the comments
consistent with the code.
arg-skipping code for powerpc dynamic linker

this allows using the dynamic linker as a command to load programs.
math: cleanup exp2.c exp2f.c and exp2l.c

* old code relied on sign extension on right shift
* exp2l ld64 wrapper was wrong
* use scalbn instead of bithacks
add stub versions of some missing optional pthread interfaces

priority inheritance is not yet supported, and priority protection
probably will not be supported ever unless there's serious demand for
it (it's a fairly heavy-weight feature).

per-thread cpu clocks would be nice to have, but to my knowledge linux
is still not capable of supporting them. glibc fakes them by using the
_process_ cpu-time clock and subtracting the thread creation time,
which gives seriously incorrect semantics (worse than not supporting
the feature at all), so until there's a way to do it right, it will
remain as a stub that always fails.
add cleaned-up sys/mtio.h

this is mostly junk, but a few programs with tape-drive support
unconditionally include it, and it might be useful.
fix typo in dynamic linker path file loading code

fortunately the memory corruption could not hurt anything, but it
prevented clearing the final newline and thus prevented the last path
element from working.
ppc socket.h: add forgotten cmsghdr
math: expf.c cleanup

similar to exp.c cleanup: use scalbnf, don't return excess precision,
drop some optimizatoins.
exp.c was changed to be more consistent with expf.c code.
math: expl.c cleanup

raise overflow and underflow when necessary, fix various comments.
math: use float constants in exp10f.c

use the 'f' suffix when a float constant is not representable
fcntl.h: O_SEARCH was missing for powerpc

put some macros that do not differ between architectures in the
main header and remove from bits.
restructure mips header so it has the same structure as the others.
Merge remote-tracking branch 'nsz/math'
fix feholdexcept -- it needs to clear exceptions after saving environment
fenv support for ppc, untested

based on code sent to the mailing list by nsz, with minor changes.
add missing const on powerpc FE_DFL_ENV
add port io functions to sys/io.h

based on proposal by Isaac Dunham. nonexistance of bits/io.h will
cause inclusion of sys/io.h to produce an error on archs that are not
supposed to have it. this is probably the desired behavior, but the
error message may be a bit unusual.