Make expand-macro.pl accept macro expressions, i.e. macros with

Make expand-macro.pl accept macro expressions, i.e. macros with
arguments. This makes it much more convenient to get expanded
expressions that can be directly copied to a debugger. This is
optional, so the original behaviour is maintained. Allow to read
the macro name or expression from stdin, which can be useful for
feeding it multi-line macro expressions. Use Pod::Usage and move
the usage to POD section.

p4raw-id: //depot/perl@34474