libsys: fix sleep(3)/usleep(3) cancel behavior

Move functions back to libc/gen sources; they are only versioned from
libc and not libsys.
Access libsys interposing slots using libsys_interposing_slot()
instead of direct libsys_interposing array dereference, which cannot
work from libc.

Reported by: glebius
Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D44042