Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r244236,r244408,r244915,r247054,r251512:

Add a new knob WITH_DEBUG_FILES to control the building of standalone
debug files for userland programs and libraries. The "-g" debug flag
is automatically applied when WITH_DEBUG_FILES is set.

The debug files are named ${prog}.debug and ${shlib}.debug for consistency with other systems and documentation. In addition they are
installed under /usr/lib/debug, to simplify the process of installing
them if needed after a crash. Users of bsd.{prog,lib}.mk outside of the
base system place the standalone debug files in a .debug subdirectory.
GDB automatically searches both of these directories for standalone
debug files.

Revert changes that ocassionally got into r241192.

Pointy hat to: me

MFC: r241002

Fix pseudo checksum calculation.

This fixes ipfilter w/ network controllers that implement only
partial rx csum offloading.

PR: 106438
Obtained from: upstream

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Add threading support.

Add basic support for PowerPC. This excludes kgdb(1).