api: spamassassin: update local channels

api: spamassassin: update local channels

This patch adds a helper to loop over all present Spamassassin
channels files in /etc/mail/spamassassin/channel.d and:
* import the included gpg key into sa-update's keyring
* run sa-update for each channel separately

the verbose argument of the helper is for reusing the code in
pmg-daily (where we only want to log errors and be less informative)

the $SA_UPDATE variable hardcoding the path of /usr/bin/sa-update was
dropped in favor of using 'sa-update' without path since we do have a
sensible setting of PATH everywhere, and hardcoding paths is
problematic (especially in usr-merged systems).

The choice of invoking sa-update for each channel separately, instead
of providing multiple '--channel' and '--gpgkey' options to a single
command was made to prevent downloading signatures, which were signed
by a key not configured for the channel.

Importing gpg-keys is also done with individual sa-update invocations,
because sa-update only imports the last present --import argument
(wrong use of Getopt::Long)

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