[compiler.h] Migrate many C++ clients of countof to fbl::count_of

The latter is safer in C++, and the former is a namespace violation in
compiler.h that eventually needs to be resolved.

Test: CQ; should be no functional change
Change-Id: I9809b9fe062fd5f07226323035bf094b3f9de798

[fs-management][minfs]: Add option to set min data slices.

Allows the FS to pre-allocate data slices when created, so it will have
a known minimum size, unless resized.

Add mkfs option to set the data slices, and a minfs option to reflect
this, so it can be propagated through fs-mkfs.

TEST=fs-management-test, fs-test

Change-Id: I3d774da9166d2949485709b913100c3113045fd3

[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[blobstore] Rename blobstore to blobfs

Also, mount blobfs at /blob rather than /blobstore.

Change-Id: Ifdc473752ede37b909b30e35dbaac1397565a659

[storage] Use c++ for fs-management' fsck, mkfs, and mount

All fs-management now uses c++ idioms.

This CL is meant to maintain the same behavior of the previous
code with one exception:

mount can now return ZX_ERR_IO or ZX_ERR_NO_MEMORY when attempting to
create the mount directory... where the previous code was returning
an incorrect status value in that case (ZX_OK).

ZX-1381 #done

Change-Id: I4e4f9cf5ff55fa534e88f4a4297d0d38ae4e4be3

[storage] Rename fs-management' fsck.c, mkfs.c and mount.c to cpp

Minimum in-file changes.

ZX-1381

Change-Id: Ic16a447194a8ec1ac73b0545d92cb3c4e19f80a0