* Improved completion support for files with spaces

Filenames with spaces, both generally and for session names, will now
complete as expected.

* Misc shell script improvements

See discussion on:
1cca9e4b72

xargs couldn't call function `dnd()` and I couldn't find a portable way
to do it, so I replaced it with a variable which I guess should work
fine.

For w/e reason the output from `which` causes problems with nnn
rendering. This commit silences the problematic lines.