CPack-FreeBSD: add a generator for FreeBSD pkg(8)

Adds an option CPACK_ENABLE_FREEBSD_PKG to allow CPack to look
for FreeBSD's libpkg / pkg(8). If this is set and the libpkg
headers and library are found (which they will be, by default,
on any FreeBSD system), then add a FreeBSD pkg(8) generator.

The FreeBSD package tool pkg(8) uses tar.xz files (.txz) with two
metadata files embedded (+MANIFEST and +COMPACT_MANIFEST).
This introduces a bunch of FreeBSD-specific CPACK_FREEBSD_PACKAGE_*
variables for filling in the metadata; the Debian generator does
something similar. Documentation for the CPack CMake-script is styled
after the Debian generator.

Implementation notes:
 - Checks for libpkg -- the underlying implementation for pkg(8) --
   and includes FreeBSD package-generation if building CMake on
   a UNIX host. Since libpkg can be used on BSDs, Linux and OSX,
   this potentially adds one more packaging format. In practice,
   this will only happen on FreeBSD and DragonflyBSD.
 - Copy-paste from cmCPackArchiveGenerator to special-case
   the metadata generation and to run around the internal
   archive generation: use libpkg instead.
 - Generating the metadata files is a little contrived.
 - Most of the validation logic for package settings is in
   CPackFreeBSD.cmake, as well as the code that tries to re-use
   packaging settings that may already be set up for Debian.
 - libpkg has its own notion of output filename, so we have
   another contrived bit of code that munges the output file
   list so that CPack can find the output.
 - Stick with C++98.

Help/manual/cmake-modules.7.rst

@@ -60,6 +60,7 @@ All Modules
    /module/CPackCygwin
    /module/CPackDeb
    /module/CPackDMG
+   /module/CPackFreeBSD
    /module/CPackIFW
    /module/CPackIFWConfigureFile
    /module/CPackNSIS

Help/module/CPackFreeBSD.rst

@@ -0,0 +1 @@
+.. cmake-module:: ../../Modules/CPackFreeBSD.cmake

Help/release/dev/cpack-freebsd-pkg.rst

@@ -0,0 +1,5 @@
+cpack-freebsd-pkg
+-----------------
+
+* CPack gained a ``FREEBSD`` generator for FreeBSD ``pkg(8)``, configured
+  by the :module:`CPackFreeBSD` module.