ruledb: properly substitute prox_vars in headers

ruledb: properly substitute prox_vars in headers

by storing the variables as perl-string (not mime-encoded, and not
utf-8 encoded), and appropriately dealing with multi-line values to
input (folding the headers and encoding as mime).

This fixes another glitch not caught by
d3d6b5dff9e4447d16cb92e0fdf26f67d9384423

the Subject was always displayed with a '?' in the end (due to the
(quoted-printable encoded) \n added).

Additionally adapt the other callsites of PMG::Utils::subst_values
where applicable.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>