mixer(8): Improve error messsages and warnings

[christos]

Summary

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks

[dev_submerge.ch]

Just some random testing, with no pcm devices I get:

# mixer -a
mixer: mixer_get_nmixers(): No error: 0
# mixer
mixer: mixer_open((null)): No error: 0

This case could be handled with a less cryptic message.

[christos]

Improve error message for mixer_get_nmixers() and mixer_open() failures.

[markj]

usr.sbin/mixer/mixer.c

70

This code should set errno to 0 before calling strtol(). Otherwise a stale errno value might be left over from a previous libc call.

[christos]

Clear errno before calling strtol().