pf_get_sport(): Prevent possible endless loop when searching for an unused nat port

This is an import of Alexander Bluhm's OpenBSD commit r1.60,
the first chunk had to be modified because on OpenBSD the
'cut' declaration is located elsewhere.

Upstream report by Jingmin Zhou:
https://marc.info/?l=openbsd-pf&m=150020133510896&w=2

OpenBSD commit message:
Use a 32 bit variable to detect integer overflow when searching for
an unused nat port. Prevents a possible endless loop if high port
is 65535 or low port is 0.
report and analysis Jingmin Zhou; OK sashan@ visa@
Quoted from: https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/net/pf_lb.c

PR: 221201
Submitted by: Fabian Keil <fk@fabiankeil.de>
Obtained from: OpenBSD via ElectroBSD
MFC after: 1 week