pfctl: lex <=, >=, and != into a single token

lex <=, >=, and != into a single token for correctness and to reduce the
lookahead in the parser
ok henning otto

Reviewed by: zlei
Obtained from: OpenBSD, deraadt <deraadt@openbsd.org>, e6e3ecf338
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46582