Combine 'th' scripts into one file.

This merges `th-clear`, `th-ls`, and `th-undo` into `th`. `th` accepts
an action argument for each one.
Add sudo checks to `sec-check`.

All relevant lines from '/etc/sudoers' are printed. 'NOPASSWD' lines are
printed separately.
Make sure /etc/sudoers is readable in `sec-check`.
Add root check to `sec-check`.

Some checks require root. This check is overridable by the '-f' flag.