portage/mail.py: Make email date, timestamp RFC-compliant Bug 520752

Apply patch from Florent Bondoux f.bondoux@free.fr
Remove no longer used time import.
Wrap the fromatedate() call with _unicode_encode() to prevent any encoding issues.

X-Gentoo-Bug: 520752
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=520752
Reviewed-by: Zac Medico
portage/mail.py: Whitespace cleanup
Fix read only detection logic

Portage should check for a read only file system relative to ROOT and
not /.

Acked-by: Zac Medico <zmedico@gentoo.org>
Acked-by: Brian Dolbec <dolsen@gentoo.org>