fix signalfd not to ignore flags

also include fallback code for broken kernels that don't support the
flags. as usual, the fallback has a race condition that can leak file
descriptors.
fix out-of-bounds access in UTF-8 decoding

SA and SB are used as the lowest and highest valid starter bytes, but
the value of SB was one-past the last valid starter. this caused
access past the end of the state table when the illegal byte '\xf5'
was encountered in a starter position. the error did not show up in
full-character decoding tests, since the bogus state read from just
past the table was unlikely to admit any continuation bytes as valid,
but would have shown up had we tested feeding '\xf5' to the
byte-at-a-time decoding in mbrtowc: it would cause the funtion to
wrongly return -2 rather than -1.

I may eventually go back and remove all references to SA and SB,
replacing them with the values; this would make the code more
transparent, I think. the original motivation for using macros was to
allow misguided users of the code to redefine them for the purpose of
enlarging the set of accepted sequences past the end of Unicode...
optimize mbrtowc

this simple change, in my measurements, makes about a 7% performance
improvement. at first glance this change would seem like a
compiler-specific hack, since the modified code is not even used.
however, I suspect the reason is that I'm eliminating a second path
into the main body of the code, allowing the compiler more flexibility
to optimize the normal (hot) path into the main body. so even if it
weren't for the measurable (and quite notable) difference in
performance, I think the change makes sense.
implement mbtowc directly, not as a wrapper for mbrtowc

the interface contract for mbtowc admits a much faster implementation
than mbrtowc can achieve; wrapping mbrtowc with an extra call frame
only made the situation worse.

since the regex implementation uses mbtowc already, this change should
improve regex performance too. it may be possible to improve
performance in other places internally by switching from mbrtowc to
mbtowc.
mbrtowc: do not leave mbstate_t in permanent-fail state after EILSEQ

the standard is clear that the old behavior is conforming: "In this
case, [EILSEQ] shall be stored in errno and the conversion state is
undefined."

however, the specification of mbrtowc has one peculiarity when the
source argument is a null pointer: in this case, it's required to
behave as mbrtowc(NULL, "", 1, ps). no motivation is provided for this
requirement, but the natural one that comes to mind is that the intent
is to reset the mbstate_t object. for stateful encodings, such
behavior is actually specified: "If the corresponding wide character
is the null wide character, the resulting state described shall be the
initial conversion state." but in the case of UTF-8 where the
mbstate_t object contains a partially-decoded character rather than a
shift state, a subsequent '\0' byte indicates that the previous
partial character is incomplete and thus an illegal sequence.

naturally, applications using their own mbstate_t object should clear
it themselves after an error, but the standard presently provides no
way to clear the builtin mbstate_t object used when the ps argument is
a null pointer. I suspect this issue may be addressed in the future by
specifying that a null source argument resets the state, as this seems
to have been the intent all along.

for what it's worth, this change also slightly reduces code size.
getifaddrs: implement proper ipv6 netmasks