Add config knob for mixer channel name (ALSA/Linux only)

In some complex ALSA setups, using PCM or Master is not the right thing
to do, so allow a custom mixer element name to be specified in the config
file for this use case.

Request and initial patch by Kurt Vanderlinden <kurt.vanderlinden@skynet.be>.

src/conffile.c

@@ -76,6 +76,7 @@ static cfg_opt_t sec_audio[] =
 #elif defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
     CFG_STR("card", "/dev/dsp", CFGF_NONE),
 #endif
+    CFG_STR("mixer", NULL, CFGF_NONE),
     CFG_END()
   };