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/eval.c

@@ -2939,7 +2939,7 @@ void ex_lockvar(exarg_T *eap)
   if (eap->forceit)
     deep = -1;
   else if (ascii_isdigit(*arg)) {
-    deep = getdigits_int(&arg);
+    deep = getdigits_int(&arg, false, -1);
     arg = skipwhite(arg);
   }
 
@@ -15775,7 +15775,7 @@ static void f_setreg(typval_T *argvars, typval_T *rettv, FunPtr fptr)
           yank_type = kMTBlockWise;
           if (ascii_isdigit(stropt[1])) {
             stropt++;
-            block_len = getdigits_long((char_u **)&stropt) - 1;
+            block_len = getdigits_long((char_u **)&stropt, true, 0) - 1;
             stropt--;
           }
           break;