Check current args entry against opt string. If it matches
exactly, take the next arg as a string; if it matches as a prefix with
an equal sign, take the remainder as a string; if value not supplied,
default value def will be given. otherwise return 0.
Check current args entry against opt string. If it matches exactly, take the next arg as a string; if it matches as a prefix with an equal sign, take the remainder as a string; if value not supplied, default value def will be given. otherwise return 0.