debuginfod-client: Add debuginfod_set_verbose_fd and DEBUGINFOD_VERBOSE

debuginfod-client: Add debuginfod_set_verbose_fd and DEBUGINFOD_VERBOSE

Introduce a new function debuginfod_set_verbose_fd which will produce
verbose output on a given file descriptor (STDERR_FILENO if the
environment variable DEBUGINFOD_VERBOSE is set) showing how the search
for a particular client query is going.

Example output:

debuginfod_find_debuginfo 1234567890
server urls "https://debuginfod.elfutils.org/ http://dbgd.usersys.com:3632/"
checking build-id
checking cache dir /home/mark/.cache/debuginfod_client
using timeout 90
init server 0 https://debuginfod.elfutils.org/
url 0 https://debuginfod.elfutils.org/buildid/1234567890/debuginfo
init server 1 http://dbgd.usersys.com:3632/
url 1 http://dbgd.usersys.com:3632/buildid/1234567890/debuginfo
query 2 urls in parallel
server response HTTP response code said error
url 0 The requested URL returned error: 404 Not Found
server response HTTP response code said error
url 1 The requested URL returned error: 404 Not Found
not found No such file or directory (err=-2)

Signed-off-by: Mark Wielaard <mark@klomp.org>