Use bash as shebang in network initscripts

Use bash as shebang in network initscripts

/bin/sh is a symlink to /bin/bash on ipfire systems. Using /bin/sh in
the scripts as shebang hurts in two ways:

1. We use features which do not work with sh as shell. This is not
   really a problem but if we rely on features of a real bash we can
   state this clearly.
2. The syntay highlighting in vim does not work without a correct
   shebang. As I want and need correct syntax highlighting I propose to
   change the shebang.

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>