patch 9.1.0441: getregionpos() can't properly indicate positions beyond eol

patch 9.1.0441: getregionpos() can't properly indicate positions beyond eol

Problem:  getregionpos() can't properly indicate positions beyond eol.
Solution: Add an "eol" flag that enables handling positions beyond end
          of line like getpos() does (zeertzjq).

Also fix the problem that a position still has the coladd beyond the end
of the line when its column has been clamped.  In the last test case
with TABs at the end of the line the old behavior is obviously wrong.

I decided to gate this behind a flag because returning positions that
don't correspond to actual characters in the line may lead to mistakes
for callers that want to calculate the length of the selected text, so
the behavior is only enabled if the caller wants it.

closes: #14838

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>