Move generation of ExtUtils::Miniperl to ext/ExtUtils-Miniperl from minimod.pl

It does increase the lines of code slightly but it replaces a bunch of
platform specific special case code in the Makefiles for *nix, Win32 and VMS
with one unified implementation. And in Perl, rather than 3+ different
languages.

This feels like the right maintainability trade-off.
Invert the build logic for miniperlmain.c and ExtUtils::Miniperl

Now ExtUtils::Miniperl has the master version of {mini,}perlmain.c and is
checked into the repository. miniperlmain.c is now generated by a script
in regen/ which uses ExtUtils::Miniperl.

Tweak ExtUtils::Miniperl::writemain() to take an optional first argument,
a reference to a file handle. This permits the regen script to use the
regen_lib.pl functions for file opening/closing/renaming and TAP generation.

For now check in ExtUtils::Miniperl minimally modified from the version
generated by the former minimod.pl. The next commit will tidy it up.
Clean up ExtUtils::Miniperl

Give it a $VERSION.
Bring the joy of strict (and warnings) to it.
Inline the C code into writemain() instead of using $head, $tail and a regex
to split the $tail into $tail1, $tail2 and $tail3.
Tweak the NAME section so that ABSTRACT_FROM parses it.
Document the updated functionality of writemain().
Move the "editor block" from miniperlmain.c to ExtUtils::Miniperl

As miniperlmain.c is now generated by ExtUtils::Miniperl (and not the other
way round), there's no reason to have an editor block in the generated file,
as it's not intended to be edited. Instead, add the "generated from" and
read-only headers to miniperlmain.c
Merge the refactoring of ExtUtils::Miniperl and {mini,}perlmain.c generation.