Fix pointer size issues with argv on VMS

The argument 'argv' in 'main' is a short pointer to a short pointer on
VMS, regardless of initial pointer size.  We must therefore make sure
that 'copy_argv' gets a 32-bit pointer for argv, and that the copied
argv is used for the rest of main().

This introduces the local type argv_t, which will have correct pointer
size in all cases (and be harmless on all other platforms) as well as
the macro Argv, which is defined as 'copied_argv' or 'argv', as the
case may be.

Reviewed-by: Andy Polyakov <appro@openssl.org>