Rewrite arg.h

This was something I wanted to do for quite a while now.

The problem with the old arg.h is that it does not allow you to call ARGF() and
EARGF() multiple times without messing the argument up. This is an
unnecessary limitation and can lead to unexpected results for people not
aware of this problem.

	ARGBEGIN {
	case 'a':
		printf("1st call: %s\n", ARGF());
		printf("2nd call: %s\n", ARGF());
		break;
	default:
		break;
	}

	$ prog -a ARG
	1st call: ARG
	2nd call: RG

This is fixed now to properly print

	$ prog -a ARG
	1st call: ARG
	2nd call: ARG

The old version also used more local variables than necessary, as the
problem can be reduced to one single local variable within the second
loop, which expresses if the argument has been consumed or not.
The use of abort() within EARGF() was a bit drastic. exit(1) should
suffice here and align with what you expect from an e*-type function.
Additionally, the formatting I used should make readability easier and
the code deduplication in the *ARGF()-macros helps with maintainability.

The license used is ISC, which is compatible with MIT/X, GPL and so forth in
case you want to use it in your project. I explicitly added the license header
to the file making it easy to just drop it in.

There are no plans to support the obsolete ARGNUM, ARGNUMF, LNGARG syntaxes.

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