utils: allow '/' inside email address localpart

utils: allow '/' inside email address localpart

The change is motivated by a report in our community forum [0], where
a mail addressed to an address containing '/' in its local-part ended
up in the quarantine.
This is permitted by RFC5322 ([1]), and, probably more relevant,
happily accepted and processed by postfix.

Once inside the quarantine (or the statistic database) the records cannot
be displayed (due to the parameter verification failure).

This leaves the user unable to delete the quarantined mail.

Apart from the quarantine and statistics the 'pmg-email-address'
format is only used in the PBSConfig and the fetchmail configuration
(both of which are available only to the admin and can be still be
edited irrespective of the set localpart).

[0]
https://forum.proxmox.com/threads/pmg-error-parameter-verification-failed-400.82353/
[1] https://tools.ietf.org/html/rfc5322

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>