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/digraph.c

@@ -1618,7 +1618,7 @@ void putdigraph(char_u *str)
       EMSG(_(e_number_exp));
       return;
     }
-    int n = getdigits_int(&str);
+    int n = getdigits_int(&str, true, 0);
 
     // If the digraph already exists, replace the result.
     dp = (digr_T *)user_digraphs.ga_data;