audio/libmatemixer: help mate_mixer_applet to save the "track to control"

- Using mate 1.8.1 installed from packages.
  Unable to set the volume straight from the slider of the Mixer Applet,
  until I right click the icon, go to preferences, and select PCM as the
  track to control. If I restart the mate_mixer_applet
  (either via kill or regular shutdown) the setting goes back to Volume.
- After the port patch the instelled libmatemixer backend works fine.
- see also:
  https://forums.freebsd.org/threads/fixing-default-audio-device-in-mate.58957/

PR:		202578
Submitted by:	Attila <agm2port@gmail.com>
Reported by:	Abilio Marques <abiliojr@gmail.com>
Reviewed by:	Walter Schwarzenfeld <w.schwarzenfeld@utanet.at>

audio/libmatemixer/files/patch-backends_oss_oss-backend.c

@@ -0,0 +1,30 @@
+--- backends/oss/oss-backend.c.orig	2017-03-12 15:28:02 UTC
++++ backends/oss/oss-backend.c
+@@ -40,6 +40,8 @@
+      * from the sndstat file, but avoid even trying that on systems where this
+      * is not needed and the file is not present */
+ #define OSS_PATH_SNDSTAT  "/dev/sndstat"
++    /* remove SOUND_MIXER_INFO on FreeBSD, use OSS_PATH_SNDSTAT */
++#undef SOUND_MIXER_INFO
+ #endif
+ 
+ #define OSS_MAX_DEVICES   32
+@@ -377,6 +379,10 @@ read_device_label (OssBackend *oss, cons
+ {
+     guint index;
+ 
++    /* default_device setting only in read_device_label_sndstat()
++     * on FreeBSD use OSS_PATH_SNDSTAT
++     */
++
+ #ifdef SOUND_MIXER_INFO
+     do {
+         struct mixer_info info;
+@@ -430,6 +436,7 @@ read_device_label_sndstat (OssBackend  *
+ 
+     while (fgets (line, sizeof (line), fp) != NULL) {
+         gchar *p;
++        g_strchomp(line); // required for g_str_has_suffix()
+ 
+         if (g_str_has_prefix (line, prefix) == FALSE)
+             continue;