libc: simplify access; libposix: let access lie

There are a few issues with Plan 9's `access`:

- it has side effects: to test the actual access (that the file
  servers can allow or deny according to complex custom rules)
  it opens and then closes the file, allocating (and disposing) the fd
- it does not work on directories, since
  - they cannot be opened for writing, despite the fact that to
    create a file in a directory you must be granted write access on
    that directory
  - they cannot be opened for execution, despite the fact that to
    access a file in a directory you must be granted execution access
    on that directory

Despite the fact that `access` (even on UNIX) is a violation of the
"tell, don't ask" principle (the access could be forbidden just after
its successful return, making subsequent `open` fail anyway), this
fact smells of a little design error in the file interface.

So, right now we choose to let the libposix's `access` lie on directories:
it will always return 0 on AWRITE and AEXEC for them, accepting that
a successive create/mkdir may fail.

However, a cleaner file API and protocol should allow a simpler `access`
to be implemented for directories too.

qa/lib/c/access.c

@@ -26,10 +26,15 @@ main(void)
 		err = "/tmp does not exists";
 	else if(access("/tmp", AREAD) != 0)
 		err = "/tmp is not readable";
+/* NOTE:
+ * In Plan 9 access(AWRITE) and access(AEXEC) in directories
+ * fail despite the actual permission of the directory.
+ *
 	else if(access("/tmp", AWRITE) != 0)
 		err = "/tmp is not writeable";
 	else if(access("/tmp", AEXEC) != 0)
 		err = "/tmp is not traversable";
+*/
 	if(err == nil){
 		print("PASS\n");
 		exits("PASS");