syscalls.master: switch to CAPENABLED flags

Switch the main syscall table to use CAPENABLED flags rather than
capabilities.conf. This avoid synchronization issues between
syscalls.master and capabilities.conf (e.g. when renaming a syscall
during development).

For now, move capabilities.conf to sys/compat/freebsd32 and use it
there. Use of sys/compat/freebsd32/syscalls.master should be replaced
by makesyscalls.lua enhancements to allow the main one to be used.

This change results in no changes to generated files after running
make sysent.

Reviewed by: kevans, emaste
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D31350