gdb: introduce debuginfod_client_up type

Introduce and use a unique pointer specialization for the
debuginfod_client type.  The deleter calls debuginfod_end to free the
client.

gdb/ChangeLog:

	* debuginfod-support.c (debuginfod_client_deleter): New.
	(debuginfod_client_up): New.
	(debuginfod_init): Return debuginfod_client_up.
	(debuginfod_source_query): Adjust.
	(debuginfod_debuginfo_query): Adjust.

Change-Id: Ie56441e123ab80b78e5311c824c162cd804f68c0