getdigits: introduce strict, def parameters

Problem:
During a refactor long ago, we changed the `getdigits_*` familiy of
functions to abort on overflow.  But this is often wrong, because many
of these codepaths are handling user input.

Solution:
Decide at each call-site whether to use "strict" mode.

fix #5555

src/nvim/change.c

@@ -1323,7 +1323,7 @@ int open_line(
           if (*p == COM_RIGHT || *p == COM_LEFT) {
               c = *p++;
           } else if (ascii_isdigit(*p) || *p == '-') {
-              off = getdigits_int(&p);
+              off = getdigits_int(&p, true, 0);
           } else {
               p++;
           }