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 r257532 (by adrian):

Fix this build for clang.

MFC r259730:

To avoid having to explicitly test COMPILER_TYPE for setting
clang-specific or gcc-specific flags, introduce the following new
variables for use in Makefiles:

CFLAGS.clang
CFLAGS.gcc
CXXFLAGS.clang
CXXFLAGS.gcc

In bsd.sys.mk, these get appended to the regular CFLAGS or CXXFLAGS for
the right compiler.

MFC r259913:

For libstand and sys/boot, split off gcc-only flags into CFLAGS.gcc.

MFC r259927:

Fix pc98 build, by also forcing COMPILER_TYPE in sys/boot/pc98/boot2's
Makefile.

Pointy hat to: dim

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

Approved by: re (implicit)

checkpoint new version

Style: NO_MAN doesn't need any value.

o move programs to separate directories and use stock Makefiles;
this simplifies including them in crunchgen images
o rename 80211* programs to wlan* as was originally intended