_p16 adds the sys-auth/pambase dependency, which conflicts with <sys-freebsd/freebsd-pam-modules-6.2-r1, which itself is unkeyworded (#210767). Bug #213166 may also have blocking status here. Locally masked for now, but shouldn't the ~x86-fbsd keyword be dropped until this is resolved?
Sorry for the spam, wrong [direct] dependency.
Newer freebsd-pam-modules, pambase, and openpam keyworded.