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/window.c

@@ -5982,7 +5982,7 @@ file_name_in_line (
         ++p;                        /* skip the separator */
       p = skipwhite(p);
       if (isdigit(*p))
-        *file_lnum = getdigits_long(&p);
+        *file_lnum = getdigits_long(&p, false, 0);
     }
   }