debuginfod-support.c: Use long-lived debuginfod_client

debuginfod-support.c: Use long-lived debuginfod_client

Instead of initializing a new debuginfod_client for each query, store
the first initialized client for the remainder of the GDB session and
use it for every debuginfod query.

In conjunction with upcoming changes to libdebuginfod, using one client
for all queries will avoid latency caused by unneccesarily setting up
TCP connections multiple times.

Tested on Fedora 33 x86_64.

gdb/ChangeLog:

	* debuginfod-support.c (debuginfod_init): Remove.
	(get_debuginfod_client): New function.