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/syntax.c

@@ -4900,7 +4900,7 @@ static char_u *get_syn_pattern(char_u *arg, synpat_T *ci)
         ci->sp_off_flags |= (1 << idx);
         if (idx == SPO_LC_OFF) {            /* lc=99 */
           end += 3;
-          *p = getdigits(&end);
+          *p = get_int_digits(&end);
 
           /* "lc=" offset automatically sets "ms=" offset */
           if (!(ci->sp_off_flags & (1 << SPO_MS_OFF))) {
@@ -4911,10 +4911,10 @@ static char_u *get_syn_pattern(char_u *arg, synpat_T *ci)
           end += 4;
           if (*end == '+') {
             ++end;
-            *p = getdigits(&end);                       /* positive offset */
+            *p = get_int_digits(&end);                       /* positive offset */
           } else if (*end == '-')   {
             ++end;
-            *p = -getdigits(&end);                      /* negative offset */
+            *p = -get_int_digits(&end);                      /* negative offset */
           }
         }
         if (*end != ',')
@@ -4980,7 +4980,7 @@ static void syn_cmd_sync(exarg_T *eap, int syncing)
         illegal = TRUE;
         break;
       }
-      n = getdigits(&arg_end);
+      n = get_long_digits(&arg_end);
       if (!eap->skip) {
         if (key[4] == 'B')
           curwin->w_s->b_syn_sync_linebreaks = n;