mixer(8): Make mute and recsrc argument parsing more robust

For the deprecated argument interfaces of the mute (1, 0, ^) and recsrc
(+, -, =, ^) controls, we only check the first character of the
argument, which means that an argument such as "vol.mute=10" would be
valid. Fix this by checking the whole argument string.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45973