Support for remounting tmp if its noexec

Some packages require /tmp to be executable when installing or upgrading. This
reuses the apt_remount_filesystem filesystem option but it might be worth
splitting them out.
Create new variable to control mounting of tmpfs

As there is every reason to think tmp will be noexec without root being read
only the options have been separated. Because of the need to clarify which is
which i've postfixed the existing option with _rootfs and added a _tmpfs
option.
refactored remount fs merge
Merge branch 'release/1.4.0'