smbd: fix cups printcap cache updates on startup

smbd: fix cups printcap cache updates on startup

On startup the parent smbd process currently calls pcap_cache_reload(),
which is done immediately before the background queue process is forked.

pcap_cache_reload() is asynchronous with cups, in that it forks a
separate process to obtain the printer listing. The cache_fd_event
print_cups.c global variable is used to track when a cups printer
listing is in progress.

cache_fd_event is set when the background queue process is forked, due
to smbd's pcap_cache_reload() call immediately prior. As a result, the
background queue process assumes an existing pcap_cache_reload() call is
indefinitely outstanding, causing the printcap cache to remain stale
thereafter.
(cherry picked from commit d7286bb6520ebe03355e98e3311e1d79e2746791)