MFC r262020:

Make it possible to use the env kernel config file option for AVILA
and CAMBRIA boards that does not use loader to load the kernel. This
is basically how it was done for i386. This way tunables can also be
set. For example in config file:

env "/conf/AVILA.env"

And in AVILA.env:

vfs.unmapped_buf_allowed=0
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@
r262400,r262401,r262418

Sync pkg(7) with head