lib: Ported 'no (enable) password' from stable/3.0

lib: Ported 'no (enable) password' from stable/3.0

The pull request #1545 from @donaldsharp introduced the command 'no
password' to remove an existing terminal connection password.
Additionally, warnings have been added to both 'no password' and 'no
enable password' to make the user aware of any security implications.

It seems that this specific pull request was never merged against master
and got lost. This commit is a cherry-pick of d4961273cb with fixed
conflicts and updated documentation.

Thanks to @donaldsharp and @pogojotz for the original PR.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>