ureport: introduce HTTPAuth

Read HTTP Basic Authentication credentials from the configuration file.

HTTPAuth has priority to SSLClientAuth. When both are set to some value
the latter is ignored.

HTTPAuth configuration option values:
 - "rhts-credentials" : a place holder for Login= and Password= from
                        rhtsupport.conf
 - "<username>:<password>"
 - "<username>" : a prompt will be issue for password (export
                  REPORT_CLIENT_NONINTERACTIVE=1 env variable to tell
                  libreport that it must not wait for user input).

Related: #1140224

Signed-off-by: Jakub Filak <jfilak@redhat.com>