close #1917: add pmg-system-report command

close #1917: add pmg-system-report command

pmg-system-report gathers information about a PMG installation, like
pvereport does for PVE.

The name was chosen because pmgreport is already taken (for the daily
reportmails).

The DNS resolution check uses SpamAssassin's internal DnsResolver, since
SpamAssassin has a few pecularities, e.g. only using the first entry in
/etc/resolv.conf - see [0] and Mail::SpamAssassin::DnsResolver and
spam-detection is abysmal if SpamAssassin cannot resolve RBL-entries.
The SpamAssassin initialization is taken from pmg-smtp-filter (except that
local_tests_only is unconditionally disabled (otherwise it would not do DNS
Resolution).

[0] https://wiki.apache.org/spamassassin/DnsBlocklists

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>