[magenta] Prevent opening directories as writable

Errno "EISDIR" for "open" can be returned for the following reason,
according to POSIX:
The named file is a directory and oflag includes O_WRONLY or O_RDWR.

(IEEE Std 1003.1, 2004 Edition)

Convert usage of opening a directory with "O_RDWR" or "O_WRONLY" to
"O_DIRECTORY" explicitly, when possible.

Change-Id: I47094798677dd22abcf2c40e3a1ff7b17525ba8c

[utest/fs] Expand rename tests, consolidate common functionality

Change-Id: Ieb39660bf9c7987f31f3ca1943efb96945ceefc8

[fs] Converted filesystem test suite from uapp --> utest

- This is possible now that we can identify "large" tests,
and prevent them from executing by default.

While I was there...
- I also fixed up the "test random op" condvar race contition
that previously plagued the concurrent "rw workers" test.
- Stdout spam reduced for the maxfile test.

Change-Id: I87fd25b6312d3490fa7842b9de22826646743d2a