add helper for parsing SA channel.d files

add helper for parsing SA channel.d files

RHEL/CentOS based SpamAssassin implementations ship an update script,
which reads shell snippets from
/etc/mail/spamassassin/channel.d/*.conf and uses the information there
to update SA rules from the configured channels [0].

Noticed the existence of this directory/mechanism while reading the
announcement of the updatechannel for the KAM ruleset [1].

Parsing the file as text, instead of sourcing it in a shell, since I
hope that the channel files distributed don't rely on running commands
to get the ruleset url and gpg key.

The parser has some minimal tests added (inspired by the
convert_size_test.pl from pve-common)

[0] https://src.fedoraproject.org/rpms/spamassassin/blob/master/f/sa-update.cronscript
[1] https://mcgrail.com/template/kam.cf_channel

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