For Unix, refactor OSSL_sleep() to use nanosleep() instead of usleep()

For Unix, refactor OSSL_sleep() to use nanosleep() instead of usleep()

usleep() is obsolete since POSIX.1-2001 and removed in POSIX.1-2008,
in favor of nanosleep(), which has been present since POSIX.1-2001.

The exceptions for DJGPP and TANDEM are preserved.  Also, just in case
nanosleep() turns out to be unavailable on any Unix machinery that we
are unaware of, we allow a revert to using usleep() by defining
OPENSSL_USE_USLEEP.

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24173)