input_stream: use "goffset" instead of "off_t"

The "off_t" type may change when you enable or disable large file
support on 32 bit platforms.  This caused severe ABI problems within
MPD when we enabled LFS for the first time: two sources included
config.h and sys/types.h in different order, and had different off_t
sizes - leading to memory corruption because of ABI incompatibility.
This patch attempts to get rid of all public "off_t" uses: it removes
"off_t" from the input_stream ABI/API, and switches to GLib's 64 bit
"goffset" type.  This may hurt 32 bit embedded platforms a tiny bit,
but that's not even measurable.

src/input/mms_input_plugin.c

@@ -110,7 +110,7 @@ input_mms_buffer(G_GNUC_UNUSED struct input_stream *is)
 
 static bool
 input_mms_seek(G_GNUC_UNUSED struct input_stream *is,
-	       G_GNUC_UNUSED off_t offset, G_GNUC_UNUSED int whence)
+	       G_GNUC_UNUSED goffset offset, G_GNUC_UNUSED int whence)
 {
 	return false;
 }