[FIX] ir.mail.server: restore parsing of multiple RFC2822 addresses including non-ASCII chars

Rev f2cf6ced1 modified RFC2822 parsing in order to better support
unicode characters inside the Name part of an address header.
However the patch broke handling of multiple addresses (comma
separated) - silently discarding all recipients except the
first one, as soon as any non-ASCII character was present.

This patch restores the functionality while preserving the
fix from f2cf6ced1, and simplifies the code using email.utils
utility functions.

Fixes (again) lp:1272610, OPW 607683