ifupdown2: fixup maintainer scripts

ifupdown2: fixup maintainer scripts

Testing Done: of course none ;) Will update when I have some time to test

fupdown2's postinst and postrm scripts do not conform to Debian's policy
for maintainer scripts [1]. Specific non-conformance follows:

* fixed maintainer scripts so they only use /bin/sh as other shells are
  non-essential packages and may not exist on the system
* fixed maintainer scripts so only actions specified by $1 are performed
  per run of the script
* fixed maintainer scripts to only support specific values for $1, and
  will otherwise exit with error
* included at the bottom of the postrm script the "#DEBHELPER#" tag as
  during package building debianhelper will inject extra code at this
  location

[1] https://www.debian.org/doc/debian-policy/ch-maintainerscripts.html
(cherry picked from commit 2289fe4e90a0134fcb07fe7e2e838de755d73e68)