toke.c: Set locale for all scan_num() calls; restore instead of reset

toke.c: Set locale for all scan_num() calls; restore instead of reset

One call of Perl_scan_num changes the locale around it.  However, this
function is called in several places, including from outside the file.
It is better to set the locale within scan_num() at the point where
it matters.  And, instead of setting the locale unconditionally, it is
better to change it only if it needs to be changed, and restore it to
the original.  Otherwise the locale can be changed to something
unexpected.