Cleanup: Refactor getdigits().

Problem  : getdigits() currently returns a long, but at most places,
           return value is casted (unsafely) into an int. Making casts
           safe would introduce a lot of fuss in the form of assertions
           checking for limits.
Note     : We cannot just change return type to int, because, at some
           places, legitimate long values are used. For example, in
           diff.c, for line numbers.
Solution : Introduce new functions:
           - get_digits()      : Gets an intmax_t from a string.
           - get_int_digits()  : Wrapper for ints.
           - get_long_digits() : Wrapper for longs.
           And replace getdigits() invocations by the appropiate
           wrapper invocations.

src/nvim/eval.c

@@ -2750,7 +2750,7 @@ void ex_lockvar(exarg_T *eap)
   if (eap->forceit)
     deep = -1;
   else if (vim_isdigit(*arg)) {
-    deep = getdigits(&arg);
+    deep = get_int_digits(&arg);
     arg = skipwhite(arg);
   }
 
@@ -13370,7 +13370,7 @@ static void f_setreg(typval_T *argvars, typval_T *rettv)
         yank_type = MBLOCK;
         if (VIM_ISDIGIT(stropt[1])) {
           ++stropt;
-          block_len = getdigits(&stropt) - 1;
+          block_len = get_long_digits(&stropt) - 1;
           --stropt;
         }
         break;