rootfspostcommands.py: Restructure sort_passwd and related functions

rootfspostcommands.py: Restructure sort_passwd and related functions

This change proposes a restructure of the functions in
rootfspostcommandstests.py to clarify the purpose of each function and
also, make it scalable for other use cases (for example adding support
for removing subid backup files).

The main function of interest here is 'tidy_shadowutils_files' which
brings in the functionality of the old 'sort_passwd' making it clear
that it doesn't only sort the passwd file:
- delete backup files
- it sorts passwd, group and the associated shadow files

The other functions are also renamed for consistency and clarity and
more documentation was added.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>