Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

MFC: r263648, r264789, r266636

This brings:
- schema validation
- xpath-like interface for ucl objects

Adapt pkg(7) to the new libucl API

MFC: r262398,r262399,r262410,r262975

Import libucl into head

UCL is heavily infused by nginx configuration as the example of a convenient
configuration system. However, UCL is fully compatible with JSON format and is
able to parse json files.

UCL is used by pkg(8) for its configuration file as well for the manifest format
in packages, it will be used in base for the pkg boostrap (signature checking
and configuration file parsing.)

libucl has been developped and is maintained by vsevolod@

Import libucl into head

UCL is heavily infused by nginx configuration as the example of a convenient
configuration system. However, UCL is fully compatible with JSON format and is
able to parse json files.

UCL is used by pkg(8) for its configuration file as well for the manifest format
in packages, it will be used in base for the pkg boostrap (signature checking
and configuration file parsing.)

libucl has been developped and is maintained by vsevolod@