toke.c, S_scan_ident(): Don't take a "end of buffer" argument, use PL_bufend

All but one of scan_ident()'s callers already passed PL_bufend as
the removed argument; The one deviant was intuit_more(), which was
setting the "end of buffer" argument, to the next close-bracket.
This commit modifies intuit_more() to temporarily set PL_bufend and
then restore it.

This was done as groundwork for the following commit, which will add
more uses of PEEKSPACE() to scan_ident() in order to fix some whitespace
and line number bugs, and PEEKSPACE() modifies PL_bufend directly
if it encounters a newline at the end of the buffer -- that last bit
being why changing intuit_more() to modify-and-restore PL_bufend is
safe, since the end of the buffer will always be a ']'
WIP