'main<>' entry point arguments and return value now optional

The 'eel' executive will now accept programs where the 'main'
entry point is declared procedure (no return value) and/or
with any argument signature, including no arguments at all.

If 'main' returns no result, the executive returns 0 to the
operating system, where applicable.

If main has no arguments, the "application path" argument is
ignored. (It's secial-cased to not cause EEL_XMANYARGS, as
that would render the "no arguments" option useless.)
However, the 'eel' executive *will* check for additional
arguments, and fail if any are found!

For any other argument signature, argument count checking
will be performed as usual, so it's entirely possible to use
this to have command line tools fail if they aren't given a
correct argument count, without explicit script code.

Closes #4.