Invert the build logic for miniperlmain.c and ExtUtils::Miniperl

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.