add -E in user executables

Add -E support in executables, for use by launchers in forwarding the environment to the user program processes when the underlying system launcher can't do so.  The changes are:

runtime/src/arg.c
  Add -E option parsing and environment variable setting.

doc/release/README.executing
  Document the new option.

test/execflags/ferguson/help2.good
test/execflags/shannon/configs/help/basehelp.txt
test/execflags/shannon/help.good
  Update the expected "-h" output.

test/execflags/gbt/dash-E-1-opt.chpl
test/execflags/gbt/dash-E-1-opt.execopts
test/execflags/gbt/dash-E-1-opt.good
test/execflags/gbt/dash-E-1-opt.skipif
test/execflags/gbt/dash-E-2-opts.chpl
test/execflags/gbt/dash-E-2-opts.execopts
test/execflags/gbt/dash-E-2-opts.good
test/execflags/gbt/dash-E-2-opts.skipif
  New unit tests.
This seems a little unconventional, in that I don't *think* any of our other flags mess with currentArg.  I think this is fine here, because it is static, but I think I'd feel better about it if the comment block were moved outside of the function, because it's more of a general comment on the interface.