socket_util: added function sockaddr_to_string()

Create the socket_util.c library, the first function is
sockaddr_to_string(): it converts a sockaddr struct to a string
containing the IP address in a human-readable form.

Makefile.am

@@ -113,6 +113,7 @@ mpd_headers = \
 	src/song_save.h \
 	src/song_sticker.h \
 	src/songvec.h \
+	src/socket_util.h \
 	src/state_file.h \
 	src/stats.h \
 	src/sticker.h \
@@ -201,6 +202,7 @@ src_mpd_SOURCES = \
 	src/song_print.c \
 	src/song_save.c \
 	src/songvec.c \
+	src/socket_util.c \
 	src/state_file.c \
 	src/stats.c \
 	src/tag.c \