Incorrect assumptions in account discovery

[mssalvatore]

Description

The Linux account discovery, Linux clear command history, and Linux modify shell startup files PBAs make incorrect assumptions about Linux users.

1. Assumes all users' home directories are in /home
2. Assumes users' home directories have the user's name. (e.g. testuser's home directory is /home/testuser)
3. Assumes regular users have 4 digit IDs.

In addition, these 3 PBAs duplicate the same command.

Proposed solution

Instead of running complicated shell commands which require shell=True (which is a security concern, see also #1118), the logic for these PBAs can be reimplemented in Python.

Correcting false assumptions

1. Rather than filtering users by the length of their ID, the code can parse the contents of /etc/passwd using the Python pwd module and return only non-root users with the following shells:
   • bash
   • sh
   • dash
   • fish
   • zsh
   • ksh
   • csh
   • tsch
2. Get each user's home directory using the pwd module instead of assuming it's /home/[USERNAME]
3. Make this code reusable (and testable)

[cakekoa]

PBAs are being removed in #2442