handle single command and remove hacks used as substitute for it

mpd-git has single command, which provides functionality
of both "repeat one" and "stop after current track" modes.

repeat one = single && repeat
stop after current track = single && !repeat

doc/keys

@@ -70,10 +70,10 @@
 #
 #key_toggle_repeat = 'r'
 #
-#key_toggle_repeat_one = 'R'
-#
 #key_toggle_random = 'z'
 #
+#key_toggle_single = 'y'
+#
 #key_shuffle = 'Z'
 #
 #key_toggle_crossfade = 'x'
@@ -136,8 +136,6 @@
 #
 #key_go_to_containing_directory = 'G'
 #
-#key_start_searching = 'y'
-#
 #key_go_to_parent_dir = 263 127
 #
 #key_switch_tag_type_list = '`'