sys: email: use `epoch_to_rfc2822` from proxmox_time

sys: email: use `epoch_to_rfc2822` from proxmox_time

`strftime`'s formatting is locale-dependent. If the system locale was
set to e.g. de_DE.UTF-8, the `Date` header became invalid
(e.g Mo instead of Mon for 'Monday'), tripping up some mail clients
(e.g. KMail).

This commit should fix this by using the new `epoch_to_rfc2822`
function from proxmox_time. Under the hood, this function uses
`strftime_l` with a fixed locale (C).

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>