Add forcedel

If the install fails (or partly succeeds), passing the "forcedel" arguments undoes any changes, so the script can start over cleanly.

Also misc fixes: better dash compatibility, put the variables at top for clarity, and an extra variable.

Closes #3
curl in wrong place
add forcedel
fix quotes (unary operator)
Merge pull request #20 from AladW/patch-1

Add forcedel