Remove svn:mergeinfo from the releng/10.0 branch.

After branch creation from stable/10, the stable/10 branch mergeinfo
was moved to the root of the branch.

Since there have not been any merges from stable/10 to releng/10.0
yet, we do not need to track any of the existing mergeinfo here.

Merges to releng/10.0 should now be done to the root of the branch.

For future branches during the release cycle, unless otherwise noted,
this change will be done as part of the stable/ and releng/ branch
creation.

Discussed with: peter
Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- 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

MFC r256450:

Rename libbsdyml to libyaml, make private, and bump
SHLIB_MAJOR to 1.0

Approved by: bapt
Approved by: re (glebius)

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

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

Add a stub manpage modeled over the libexpat one to explain to users not to use
this library outside of base.

Requested by: simon

Import libyaml as libbsdyml (private brand name)

LibYAML is a YAML 1.1 parser and emitter under MIT license which will
soon be used by the pkg boostrap (usr.bin/pkg) and bhyve

Reviewed by: roberto, antoine