mixer(8): Report an error if passed value is an empty string

[0mp]

Summary

mixer(8): Report an error if passed value is an empty string

This patch fixes a bug that made the mixer command enter an infinite loop
when instructed to set the value of a device to an empty string (e.g., `mixer vol ""`).

Additionally, some tests for mixer(8) are being added.

PR:	240039

• https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240039

Test Plan

$ sudo mkdir -p /usr/tests/usr.sbin/mixer/
$ cd src/usr.sbin/mixer
$ make && sudo make install && (cd /usr/tests/usr.sbin/mixer/ && kyua test)

[0mp]

Fix a typo in an atf_test description

[mav]

Looks good to me. Just thinking whether -1 or EOF is more appropriate.

[0mp]

In D21409#465784, @mav wrote:

Looks good to me. Just thinking whether -1 or EOF is more appropriate.

Probably EOF. I'll change it in the final commit.

Also, do we want to MFC this change?

[mav]

In D21409#465864, @0mp wrote:

Also, do we want to MFC this change?

Probably. Why not.