fix #2111: regex match for email addresses

now allows:
* addresses without tld (f.e. user@localhost per bug)
* remove limits for number of subdomains
* allow +, -, ~ in local part
* disallow double dots (.. .a. etc) and dots in the end (abc.@mail.com)

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Acked-by: Dominik Csapak <d.csapak@proxmox.com>