[minfs] Pre-reserve inodes/blocks to mitigate mid-op failures.

The most common failure for a Minfs operation (in particular for
writes) occurs when we attempt but are unable to allocate a new
block or inode. To ensure that this kind of error cannot happen
partway through an operation and leave the in-memory metadata in
an inconsistent state, the maximum number of blocks required for
an operation will be "reserved" prior to any state-changing calls.

In many cases the reserved elements will be an over-estimated, in
which case they will be unreserved at the end of the operation.

Additionally, in cases where a directory entry must be appended in
conjunction with other updates, ensure that the directory has
space for the new entry before making any other changes.

Test: TestFullOperations in utest/fs/test-minfs

Change-Id: I16bad01344723dc6b1dddfd2a04c919846032597

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[fs] Expand persistence test

- Convert to C++
- Test persistent directories
- Test persistent files, with varying buffer sizes
- Test renaming a "target" file in a loop, verifying
that it exists in exactly one spot in loop after reboot.

Change-Id: I017041485cd4319fc740487dda521dcb09f0f613

[mxio][fs] Add support for rewinddir

- Also restructured the dircookie in MinFS to avoid
overflowing the contained "vdircookie_t" in iostate.
Included a static assertion to avoid this problem
in future patches.
- Test added for rewinddir.

US-124 #done

Change-Id: I8c4af45a62f27193bd040fb77ec372b9ebb19ec2

[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