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/search.c

@@ -4611,7 +4611,7 @@ int read_viminfo_search_pattern(vir_T *virp, int force)
     if (lp[4] == 'E')
       off_end = SEARCH_END;
     lp += 5;
-    off = getdigits(&lp);
+    off = get_long_digits(&lp);
   }
   if (lp[0] == '~') {           /* use this pattern for last-used pattern */
     setlast = TRUE;