server/audiod: Don't parse loglevel argument unnecessarily.

server/audiod: Don't parse loglevel argument unnecessarily.

Currently the severity string (debug, info, etc.) given to --loglevel
is parsed twice: Once by lopsub, which returns the loglevel as the
index into the array of severity strings. We turn this index into a
string and pass the string to daemon_set_loglevel() which parses the
string again to turn it back into a log level value (which happens
to coincide with the index value).

Clean this up by letting daemon_set_loglevel() receive a log level
value rather than a severity string. This also allows us to remove
the now unused ENUM_STRING_VAL() macro from audiod.c.