make realpath use O_PATH when opening the file

this avoids failure if the file is not readable and avoids odd
behavior for device nodes, etc. on old kernels that lack O_PATH, the
old behavior (O_RDONLY) will naturally happen as the fallback.

src/misc/realpath.c

@@ -21,7 +21,7 @@ char *realpath(const char *restrict filename, char *restrict resolved)
 		return 0;
 	}
 
-	fd = open(filename, O_RDONLY|O_NONBLOCK|O_CLOEXEC);
+	fd = open(filename, O_PATH|O_NONBLOCK|O_CLOEXEC);
 	if (fd < 0) return 0;
 	__procfdname(buf, fd);