Use dhcpcd -k to force the release, instead of SIGHUP

Newer dhcpcd versions use a different signal internally (SIGALRM), but
dhcpcd -k interface_name will work with both newer and older versions
of dhcpcd.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Fix https://bugs.gentoo.org/show_bug.cgi?id=500942

“unset x” also removes the “local x” property, so the next assignment and/or read to/from $x will access the global variable “x”.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>