Makefile: Add missing entry for archive-imap

Update the Makefile to install archive-imap.py
to bin/archive-imap.
Remove execution permission from files
mkpasswd: Store original code as mkpasswd.py

This commit adds the suffix .py to the mkpasswd
file. The suffix is stripped when installing
the file.
mkpasswd: Use python2 for execution

Some distribution do not ship a "python"
executable. Use "python2" instead.