Switch to upstream sleep(3) and usleep(3).

Also fix the signature of usleep, and the definition of useconds_t which
should be unsigned, as the 'u' in its name implies.

This patch also cleans up the existing FreeBSD hacks by moving the libm
stuff from <sys/cdefs.h> to a libm-private header, and adding comments
about the hacks we use to build FreeBSD source.

Change-Id: Ibe5067a380502df94a0a3a7901969b35411085b6