Disallow cap_enter() in freebsd32 compatibility mode.

Disallow cap_enter() in freebsd32 compatibility mode.

The freebsd32 compatibility mode (for running 32-bit binaries on 64-bit
kernels) does not currently allow any system calls in capability mode, but
still permits cap_enter(). As a result, 32-bit binaries on 64-bit kernels
that use capability mode do not work (they crash after being disallowed to
call sys_exit()). Affected binaries include dhclient and uniq. The latter's
crashes cause obscure build failures.

This commit makes freebsd32 cap_enter() fail with [ENOSYS], as if capability
mode was not compiled in. Applications deal with this by doing their work
without capability mode.

This commit does not fix the uncommon situation where a 64-bit process
enters capability mode and then executes a 32-bit binary using fexecve().

This commit should be reverted when allowing the necessary freebsd32 system
calls in capability mode.

Reviewed by:	pjd
Approved by:	re (hrs)