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_stream.c

@@ -139,7 +139,7 @@ input_stream_open(struct input_stream *is, const char *url)
 }
 
 bool
-input_stream_seek(struct input_stream *is, off_t offset, int whence)
+input_stream_seek(struct input_stream *is, goffset offset, int whence)
 {
 	if (is->plugin->seek == NULL)
 		return false;