[gdb] Print progress for debuginfod

Prints progress like:

Downloading 4.89 MB separate debug info for /usr/lib64/libgcrypt.so.20.
Downloading 1.10 MB separate debug info for /usr/lib64/liblzma.so.5.
Downloading 1.31 MB separate debug info for /usr/lib64/liblz4.so.1.
Downloading 0.96 MB separate debug info for /usr/lib64/libsmime3.so.
[###                                                                    ]

Tested on x86_64-linux.

ChangeLog:

2020-12-16  Martin Liska  <mliska@suse.cz>
	    Tom de Vries  <tdevries@suse.de>

	* gdb/debuginfod-support.c (struct user_data): Remove has_printed
	field.  Add meter field.
	(progressfn): Print progress using meter.