alsa: fix that r/w permissions on /dev/snd/seq were not being checked

Author: jcelerier

include/libremidi/backends/alsa_seq.hpp

@@ -42,7 +42,7 @@ struct backend
     if (!snd.available || !snd.seq.available)
       return false;
 
-    return ::access("/dev/snd/seq", F_OK) == 0;
+    return ::access("/dev/snd/seq", F_OK | R_OK | W_OK) == 0;
   }
 };
 

include/libremidi/backends/alsa_seq_ump.hpp

@@ -63,7 +63,7 @@ struct backend
     if (!snd.available || !snd.seq.available || !snd.seq.ump.available || !snd.ump.available)
       return false;
 
-    return ::access("/dev/snd/seq", F_OK) == 0;
+    return ::access("/dev/snd/seq", F_OK | R_OK | W_OK) == 0;
   }
 };
 }