Refine the "usage" portion of arg.cpp


a) Did some simple "cleanup" of usage() in main/arg.cpp

b) usage() has an option that prints the current value of the variables
that are referenced by the ArgumentDescription[].  Tweaked to handle
the case in which these are NULL

Compiled:   STD on clang   STD/LLVM on gcc on chap01
Tested:     Inspected the usage for a few test cases
Reviewed:   None




git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23611 3a8e244f-b0f2-452b-bcba-4c88e055c3ca