websocket: Fix a return value for parse failures

Fix return value for web_socket_util_parse_req_line() so that
it can return negative values on parse failures.

Closes #1547
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
tools: Bring in changes from Fedora and RHEL cockpit.spec

 * Build cockpit-docker as a separate package
 * Have cockpit-shell depend on the NetworkManager, and various
   other system services.

Closes #1555
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
 * Tweak testsuite-prepare to not filter our cockpit-docker
   package by accident.