MFC r312452-r312512:

r312452-r312512:

- Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This simplifies pathing in make/displayed output

- Use .CURDIR:H instead of .CURDIR to simplify pathing in output, etc

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Merge the projects/release-pkg branch to head.

This allows packaging the base system with pkg(8), including
but not limited to providing the ability to provide upstream
binary update possibilities for non-tier-1 architectures.

This merge is a requirement of the 11.0-RELEASE, and as such,
thank you to everyone that has tested the project branch.

Documentation in build(7) etc. is still somewhat sparse, but
updates to those parts will follow.

Sponsored by: The FreeBSD Foundation

Rework PRIVATELIB

Now when a lib is marked as PRIVATELIB it is renamed into libprivate$foo instead
of being installed in /usr/lib/private and playing with rpath.

Also allow to install headers for PRIVATELIBS in that case the headers will be
installed in /usr/include/private/$foo

Keep the headers under a private namespace to prevent third party build system
to easily find them to ensure they are only used on purpose.

This allows for non base applications to statically link against a library in
base which is linked to a privatelib

Treating PRIVATELIBS as regular libraries allows to push them into our current
compatX packages if needed.

While here finish promotion of libevent as PRIVATELIB
Install header for bsdstat and libucl

Differential Revision: https://reviews.freebsd.org/D2365
Reviewed by: brooks, des
Discussed with: imp

Promote libevent to lib/ level and fold ftp-proxy into its parent Makefile.
This allows us to use libevent for other application in the future.

For now libevent is still INTERNALLIB and no shared library is installed.

MFC after: 1 month

The last big commit: let usr.sbin/ use WARNS=6 by default.

Link pf 4.1 to the build:
- move ftp-proxy from libexec to usr.sbin
- add tftp-proxy
- new altq mtag link

Approved by: re (kensmith)