utils: return perl string from decode_rfc1522

utils: return perl string from decode_rfc1522

decode_rfc1522 is a more robust version of decode_mimewords (in
scalar context) - adapt it to return a perlstring, under the
assumption that data is utf-8 encoded (holds true for ascii and
smtputf8 mails)

the try_decode_utf8 helper sub backwards will be used extensively in
later patches and is inspired by commit
43f8112f0bb424f99057106d57d32276d7d422a6 in pve-storage:
We consider that the valid multibyte utf-8 characters do not really
yield sensible combinations of single-byte perl characters (starting
with a byte > 127 - e.g. "£") so if something decodes without error
from utf-8 it will in all likelyhood have been utf-8 to begin with

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