Refactor to be sudoers-friendly (= rebase #87)

[hseg]

Rebased version of #87.
Patch reduces the set of commands to whitelist down to one.
Also, it changes the calling convention used by the PM unit from setting an
envvar to passing a fixed string.

Some misc shellcheck warnings are also cleaned up, most significantly:

• quoting in the dry run code
• quoting in the command line construction
• the fresh display search code (avoiding parsing ls output)

Note: implementation nonessentially uses Bash 4 features.
With minor duplication, older bashes can be supported.

[Nathoufresh]

Great work!
It works well on my manjaro install I use it daily.

For those who wanna try here's a PKGBUILD.
And don't forget to add %users ALL=(root) NOPASSWD:/usr/bin/nvidia-toggle at the end of you sudoers file.