sys: add conf/std.debug, generic debugging options

The new sys/conf/std.debug contains the list of debugging options
enabled by default in -CURRENT, so they don't need to be listed
individually in every kernel config.

The enabled options are the set of all debug options which were enabled
for the GENERIC kernel on any platform. This means some architectures
now have debugging options enabled in GENERIC which weren't previously
enabled:

- amd64: [1]
- arm64: [2]
- arm: [2]. [3]
- i386: [1], [2]
- powerpc: [1], [2], [3]
- riscv: [2]

[1] ALT_BREAK_TO_DEBUGGER is now enabled.
[2] BUF_TRACKING, FULL_BUF_TRACKING, and QUEUE_MACRO_DEBUG_TRASH are now
enabled.
[3] DEADLKRES is now enabled.

While here, move the documentation for the (commented out) K*SAN options
for amd64 from GENERIC to NOTES.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1124