pf: fix ICMP type/code representation

internal representation of icmp type/code in pfctl(8)/pf(4) does not
fit into u_int8_t. Issue has been noticed and kindly reported by
amalinin _at_ bh0.amt.ru via bugs@.

OK bluhm@

Obtained from: OpenBSD, sashan <sashan@openbsd.org>, 1fdb608f55
Sponsored by: Rubicon Communications, LLC ("Netgate")