Added ShellVariables.h/c

These files are meant to provide the ability to manipulate shell
variables from within electron
Fixed compilation errors in ShellVariables.c
Updated make and mk files to include ShellVariables
userfunctions.c now includes ShellVariables as part of the command set
added -lrt to the Makefile
get-shell-variable now returns false on failure instead of nil

I also fixed the other shell variable functions to follow how booleans
are supposed to work.