improve headers to better deal with removed-in-posix-2008 features

improve headers to better deal with removed-in-posix-2008 features

with this patch, setting _POSIX_SOURCE, or setting _POSIX_C_SOURCE or
_XOPEN_SOURCE to an old version, will bring back the interfaces that
were removed in POSIX 2008 - at least the ones i've covered so far,
which are gethostby*, usleep, and ualarm. if there are other functions
still in widespread use that were removed for which similar changes
would be beneficial, they can be added just like this.