dhclient: fix pcp:pcp cleanup failure

If isc-dhcp44-server is not installed (as is the case the project's CI
servers) we don't create ngctl.shutdown, causing the 'rm' to fail.
That in turn causes the cleanup function to return non-zero, which
causes kyua to consider the test failed.

Use 'rm -f' instead, so we don't error even if the file doesn't exist.

Sponsored by: Rubicon Communications, LLC ("Netgate")

dhclient: test against a real (ISC) dhcp server

This tests both both normal (untagged), and pcp tagged (VLAN 0)
networks which are now common for fiber ISPs.

Reviewed by: markj, kp
Differential Revision: https://reviews.freebsd.org/D35178