Don't use errno constants for os_open() errors

In Windows we can't assume errno will be set by calls to os_* functions,
instead the return value from os_* functions can be used. This commit fixes two
occurences for os_open().

1. EFBIG is replaced with UV_EFBIG and checked against the return from os_open().
2. EOVERFLOW does not have a corresponding libuv constant, and is not defined
   by open() in Windows - disabled this case with a UNIX guard, and check the return
   value against -EOVERFLOW (libuv errors are negative errno values in Unix).

src/nvim/fileio.c

@@ -578,11 +578,11 @@ readfile (
       return OK;                  /* a new file is not an error */
     } else {
       filemess(curbuf, sfname, (char_u *)(
-# ifdef EFBIG
-            (errno == EFBIG) ? _("[File too big]") :
-# endif
-# ifdef EOVERFLOW
-            (errno == EOVERFLOW) ? _("[File too big]") :
+            (fd == UV_EFBIG) ? _("[File too big]") :
+# if defined(UNIX) && defined(EOVERFLOW)
+            // libuv only returns -errno in Unix and in Windows open() does not
+            // set EOVERFLOW
+            (fd == -EOVERFLOW) ? _("[File too big]") :
 # endif
             _("[Permission Denied]")), 0);
       curbuf->b_p_ro = TRUE;                  /* must use "w!" now */