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/ex_cmds2.c

@@ -570,7 +570,7 @@ static int dbg_parsearg(char_u *arg, garray_T *gap)
   if (here) {
     bp->dbg_lnum = curwin->w_cursor.lnum;
   } else if (gap != &prof_ga && ascii_isdigit(*p)) {
-    bp->dbg_lnum = getdigits_long(&p);
+    bp->dbg_lnum = getdigits_long(&p, true, 0);
     p = skipwhite(p);
   } else {
     bp->dbg_lnum = 0;