Fix handling of undef, ref, typeglob, UTF8, COW and magic scalar argu…

…ment in all XS functions

Before this patch every function XS function did it differently and not
every one correctly. Now SvPV_force_nomg() is used when source argument
is going to be modified. SvGETMAGIC() is called when entering into
functions and then only "nomg" variants of perl functions are used to
prevent processing get magic more times. SvSETMAGIC() is called after
modification of source argument.

This fixes bugs:
https://rt.cpan.org/Public/Bug/Display.html?id=117158
https://rt.cpan.org/Public/Bug/Display.html?id=85489
#66