Add Archlinux to list of supported distros

There is a bug with `platform.linux_distribution()`, which returns an
empty value on Archlinux with python2.

This bug is fixed in python3, but *will not* be fixed in python2.

This patch fixes that issue with a dirty hack: on archlinux, there is a
file that can be used to detect an archlinux machine. that file is
`/etc/arch-release`. if the file exists, then the OS variable will be
set to "linux-arch".

You can learn more about that issue on the python bug platform:
https://bugs.python.org/issue20454

Signed-off-by: Philippe Loctaux <loctauxphilippe@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/utils/distro.py

@@ -49,6 +49,8 @@ def get_os_name():
         DISTRO = platform.linux_distribution()[0]
         if DISTRO:
           OS = OS + "-%s" % DISTRO.split()[0].lower()
+        if os.path.exists('/etc/arch-release'):
+            OS = "linux-arch"
 
     return OS