cleanup logic in S_sv_uncow

I couldnt understand this code at first glance. Many empty conditional
blocks because of page protection ROing which isn't implemented on Win32,
caused erratic stepping in my debugger with CC optimizations *off*.

-read COWREFCNT only once
-save COWREFCNT to an auto for debugger visual inspection
-don't check len twice
-don't write sv_buf_to_rw() twice

No change in behavior is expected.
dont test for bootstrap file twice in XSLoader

This saves ~8ms on my mech HD Win64 system per load() call.
See [perl #123373] for details.
Increase $Data::Dumper::VERSION to 2.155
Increase $mro::VERSION to 1.17
Increase $NDBM_File::VERSION to 1.14