pf: import pf_set_protostate() from OpenBSD

to change a state's state (that term is overloaded in pf, protocol state
like ESTABLISHED for tcp here), don't do it directly, but go through a newly
introduced pf_set_protostate()

Reviewed by: kbowling
Obtainted from: OpenBSD
MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31729