* Make empty string check more consistent
Replace explicit empty string check condition with inbuilt POSIX compliant empty string check flag, in adherence with the rest of the script.
* Update test expression
* Remove misleading character escape.
Everything is in single quotes, so shell will escape it correctly
without backslashes. Fixes#440.
* Remove lsblk type restriction.
* Make it possible to cancel with ESC in dmenu.
Up to now if you pressed ESC to cancel the program would just continue
with incorrect data.
