debian: Fix building Debian packages

This commit fixes a myriad of problems with Debian packaging and
attempts to remove lintian warnings.

* Update to modern debhelper (which makes the control file simpler)
  * simplify the control file
  * declare source format
  * update standards version

* Make /var/run at runtime.  /var/run may be on a temporary file
  system cleared out on reboot, so it’s necessary to re-create it
  every time the daemon is restarted.

* Correct location of mfsmount man page as mfsmount man page is in
  section 1, not 8.

* Depend on Python where necessary (mfs-cgi and mfs-cgiserv)

* Don’t Build-Depends on libc6-dev.  According to the Debian Policy
  Manual, section 4.2, you don’t need to declare a Build-Depends
  relationship on any packages in build-essential (unless you need a
  specific version, which we don’t).

* Remove conffile configuration files.  Files put in /etc are already
  marked as conffiles, so there’s no need to declare them as such.
  Moreover, putting them both in *.conffiles files and in *.install
  files the files to be duplicated in the manifest, which is never a
  good thing!

* Depend on adduser in mfs-common.  The postinst script for mfs-common
  calls adduser, so we better depend on it!

Change-Id: Idff4445a9c568b26cac9124029074d72479b87cd