'if' module: clarify documentation and test more thoroughly.

'if' module: clarify documentation and test more thoroughly.

The documentation for 'if' made certain claims about the need to quote or not
quote a module name preceding a "fat arrow" ('=>') operator.  These claims
were shown to be unfounded in most cases when "use strict 'subs'" was in
effect.

In the course of writing better documentation, it was observed that the "no
if" case was very under-tested, poorly documented and hence poorly understood.
Hence, more tests have been added and the documentation has been extensively
revised.  However, there have been no changes in source code or functionality.

Make porting/podcheck.t happy.  Compensate for functions not available on
older perls.  Documentation touch-ups suggested by sisyphus.

For:  RT # 132732.