Fix for compiling src/unlinkd.cc with kqueue and epoll.

Fix for compiling src/unlinkd.cc with kqueue and epoll.

When Squid is compiled with --enable-kqueue or --enable-epoll, we're
not supposed to use any fd_set structures.  unlinkd.cc uses select()
to pause and wait for for feedback from the external unlinkd helper.
But when using kqueue or epoll, unlinkd.cc will have to use "usleep"
emulation rather than select.

The contents of lib/xusleep.c and include/xusleep.h have been copied
from Squid-2 sources.