Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

The FP status register allows for 6 traps to be masked. One of them,
the denormal/unnormal trap, is not a standard IEEE trap. We did
not exclude it from being returned by fpgetmask(), nor did we make
sure that fpsetmask() didn't clobber it. Since the non-IEEE trap
is not part of fp_except_t, users of ifpgetmask()/fpsetmask() would
be confronted with unexpected behaviour, one of which is a SIGFPE
for denormal/unnormal FP results.

This commit makes sure that we don't leak the denormal/unnormal mask
bit in fp_except_t and also that we don't clobber it.

o There are 6 trap disable bits in ar.fpsr, not five. Even though we
didn't provide a constant for one of them (non-IEEE denormal trap),
in an attempt to not support it probably, it's not we are left with
the lower 5 bits.
o Properly mask the passed or returned fp_except_t. Not doing so
causes instant core dumps by trying to write an invalid value to
ar.fpsr. Now that we're masking, stop using exclusive-or to invert
bits.

This fixes the illegal instruction fault encountered when building
mozilla.

Fix the style of the SCM ID's.
I believe have made all of libc .c's as consistent as possible.

Implement fpsetmask() and fpgetmask().