MFC r340424:

Use the main capabilities.conf for freebsd32.

Allow the location of capabilities.conf to be configured.

Also allow a per-abi syscall prefix to be configured with the
abi_func_prefix syscalls.conf variable and check syscalls against
entries in capabilities.conf with and without the prefix amended.

Take advantage of these two features to allow use shared
capabilities.conf
between the default syscall vector and the freebsd32 compatability
layer. We've been inconsistent about keeping the two in sync as
evidenced by the bugs fixed in r340294. This eliminates that problem
going forward.

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17932