sndctl(8): Allow read-only sysctls

[christos]

Summary

Needed by follow-up patch.

Sponsored by: The FreeBSD Foundation
MFC after: 1 day

[markj]

usr.sbin/sndctl/sndctl.c

676

For such simple code, it's IMO better to just use an if-statement:

if (arg != NULL) {
    n = strtol(...)
    ...
}

679

For this to work correctly, you need to set errno = 0 before the strtol() call.

[christos]

Address Mark's comments.