printing: only reload printer shares on client enum

printing: only reload printer shares on client enum

Currently, automatic printer share updates are handled in the following
way:
- Background printer process (BPP) forked on startup
- Parent smbd and per-client children await MSG_PRINTER_PCAP messages
- BPP periodically polls the printing backend for printcap data
	- printcap data written to printer_list.tdb
	- MSG_PRINTER_PCAP sent to all smbd processes following update
- smbd processes all read the latest printer_list.tdb data, and update
  their share listings

This procedure is not scalable, as all smbd processes hit
printer_list.tdb in parallel, resulting in a large spike in CPU usage.

This change sees smbd processes only update their printer share lists
only when a client asks for this information, e.g. via NetShareEnum or
EnumPrinters.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10652

Suggested-by: Volker Lendecke <vl@samba.org>
Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 4f4501ac1f35ab15f25d207c0d33e7c4d1abdf38)