pfctl: void functions and exit(3) on error

Changes:
voided:

• pfctl_do_clear_tables
• pfctl_show_tables
• pfctl_show_ifaces

Those functions now exit(3) in case of error instead
of passing it up to the callers (where it was ignored).

OK mikeb@, sashan@

Obtained from: OpenBSD, awolk <awolk@openbsd.org>, c5e9690e42
Sponsored by: Rubicon Communications, LLC ("Netgate")