Return libuv error code from os_getperm()

Previously os_getperms() returned -1 for any error condition, it
now returns the libuv error code (as returned by os_stat()). This
allows checking for error conditions without relying on errno
(which not available in Windows).

The only case where the errno value from os_getperms() was being used
was in readfile() to set the new-file flag - replaced the errno check
with UV_ENOENT.

src/nvim/fileio.c

@@ -535,11 +535,7 @@ readfile (
     if (!newfile) {
       return FAIL;
     }
-    if (perm < 0
-#ifdef ENOENT
-        && errno == ENOENT
-#endif
-        ) {
+    if (perm == UV_ENOENT) {
       /*
        * Set the 'new-file' flag, so that when the file has
        * been created by someone else, a ":w" will complain.

src/nvim/os/fs.c

@@ -217,15 +217,16 @@ static int os_stat(const char *name, uv_stat_t *statbuf)
 
 /// Get the file permissions for a given file.
 ///
-/// @return `-1` when `name` doesn't exist.
+/// @return libuv error code on error.
 int32_t os_getperm(const char_u *name)
   FUNC_ATTR_NONNULL_ALL
 {
   uv_stat_t statbuf;
-  if (os_stat((char *)name, &statbuf) == kLibuvSuccess) {
+  int stat_result = os_stat((char *)name, &statbuf);
+  if (stat_result == kLibuvSuccess) {
     return (int32_t)statbuf.st_mode;
   } else {
-    return -1;
+    return stat_result;
   }
 }