libroot: introduce crypt_r

Change-Id: Ieacd1b383ac078a440227c7954f5531a36fbbd62
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5827
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>