lib: Add parse_one_of(), parse_on_off()

lib: Add parse_one_of(), parse_on_off()

Take from the macsec code parse_one_of() and adapt so that it passes the
primary result as the main return value, and error result through a
pointer. That is the simplest way to make the code reusable across data
types without introducing extra magic.

Also from macsec take the specialization of parse_one_of() for parsing
specifically the strings "off" and "on".

Convert the macsec code to the new helpers.

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@gmail.com>