MFC r282213:

Add kern.racct.enable tunable and RACCT_DISABLED config option.
The point of this is to be able to add RACCT (with RACCT_DISABLED)
to GENERIC, to avoid having to rebuild the kernel to use rctl(8).

MFC r282901:

Build GENERIC with RACCT/RCTL support by default. Note that it still
needs to be enabled by adding "kern.racct.enable=1" to /boot/loader.conf.

Note those two are MFC-ed together, because the latter one changes the
name of RACCT_DISABLED option to RACCT_DEFAULT_TO_DISABLED. Should have
committed the renaming separately...

Relnotes: yes
Sponsored by: The FreeBSD Foundation