libbsm: Rename package to audit-lib

This is part of audit, so use the new LIB_PACKAGE feature to name the
package audit-lib, rather than libbsm.

MFC after: 1 day
Reviewed by: bapt
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52789