blib.pm dont shell out to "cmd.exe" on Win32+Win32.pm

[bulk88]

-document Internals::getcwd() enough, with scary warnings, for future core devs, or CPAN devs. A permanent reason for Internals::getcwd() to exist on Win32 full perl was found. See code comments. -optimize a bit the built-in perl core cwd() XSUBs, use TARG, and group stack manipulation together for C compiler variable liveness reasons aka less variables to save in non-vol regs or on C stack around function calls.

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• This set of changes requires a perldelta entry, and it is included.