sendmail-helper: only send multipart if necessary

sendmail-helper: only send multipart if necessary

PVE::Tools::sendmail currently always sends a multipart/alternative message
irrespective of the actual content of the mail (e.g. a plain-text only mail
need not be sent as multipart message).

Additionally a few small refactorings based on the discussion
in https://lists.proxmox.com/pipermail/pbs-devel/2020-August/000423.html
and commited in 66004f22c6475ceb0146cf2df1f380f9f0274be4 in the
rust proxmox repository git://git.proxmox.com/git/proxmox.git
were carried over.

tested by creating a backup of a VM and setting an e-mail address, having
ha-manager send a mail after fencing as well as sending a few small mails via
'perl -e'

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