- POSIX states the SHELL environment variable "... shall represent a
pathname of the user's preferred command language interpreter." As
such, st should check for its presence when deciding what shell to
use; just as HOME can be defined to override one's passwd-defined home
directory, a user should also be able to override their passwd-defined
shell using the SHELL environment variable.
rmul means "exit underline mode", so a full reset of all
the attributes is not the correct way of exiting from
underline mode, because it is going to modify also another
attributes not related.