Prevent user from disabling the screen lock.

[adelton]

Description:

• Prevent user from disabling the screen lock.

Rationale:

• Not listing tmux among permitted shells prevents malicious program running as user from lowering security by disabling the screen lock.

[adelton]

Rebased on master -> d33a304.

[jan-cerny]

@adelton You usually add rules also to Fedora profile, why not this time?

[adelton]

The configure_bashrc_exec_tmux (which this one is copy of) did not have it either. That one is a copy of configure_tmux_lock_after_time which is a copy of configure_tmux_lock_command which finally is a rule that I did not author, and it does not have Fedora as well.

[mildas]

People can tailor Fedora profile with these tmux rules. If we would want them in, we can add them all together. Right now it would be mess, if we add this rule to the profile and other tmux rules wouldn't be there.
This PR LGTM and if you want tmux rules in Fedora profile, add them all together in new PR. Thank you