debuginfod: Use auto-sized connection pool when -C is not given with arg

debuginfod: Use auto-sized connection pool when -C is not given with arg

Since commit 4b42d9ad, libmicrohttpd's epoll event loop is used when
available in which case we must disable its setting for spawning a thread
per request.  This contradicts the debuginfod doc's description of '-C',
which indicates that if this command line option is not given then the
thread pool size is unbounded.

Fix this by using an auto-sized thread pool when '-C' is not given, just
as we do when it's given with no argument. Update the doc's description
of '-C'.

Also use a fixed-size pool even if epoll is not supported. The unbounded
pool config cannot be considered entirely reliable as it appears to cause
random fails in the run-debuginfod-webapi-concurrency test.

Signed-off-by: Aaron Merey <amerey@redhat.com>