Add function for getting current arg count
Add function for clearing all shifts
Allow setting a "base" offset that is unaffected by shifts

This is to accommodate the need to ignore the msh argument when
processing a script file
Add the $# argument count expansion
Extend fileargs test to test $# expansion
Implement shift and unshift builtins
Replace identical shift and unshift functions with a HOF
Shamelessly steal the showshift test script from Nelson