- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFprojects/camlock r254895:
Add unmapped BIO support to GEOM ZERO if kern.geom.zero.clear is cleared.

Mark all SYSCTL_NODEs static that have no corresponding SYSCTL_DECLs.

The SYSCTL_NODE macro defines a list that stores all child-elements of
that node. If there's no SYSCTL_DECL macro anywhere else, there's no
reason why it shouldn't be static.

In g_zero_destroy_geom(), return 0 instead of EBUSY in the success case.
EBUSY was probably used as a workaround for the deadlock fixed in r207671.

Approved by: pjd
X-MFC after: r207671

Remove trailing spaces.

Move $FreeBSD$ from comment to __FBSDID().

- Allow to specify the byte which will be used for filling read buffer.
- Improve sysctl description a bit.

Submitted by: Ivan Voras <ivoras@gmail.com>

Add a very simple and small GEOM class - ZERO.
It creates very huge provider (41PB) /dev/gzero.
On BIO_READ request it zero-fills bio_data and on BIO_WRITE it does nothing.
You can also set kern.geom.zero.clear sysctl to 0 to do nothing even for
BIO_READ.

I'm using it for performance testing where it is very helpful.

MFC after: 3 days