Update jobs.mk to leverage -jC

With bmake-20230909 we can use -j1.5C to use 1.5*ncpu
bmake will set .MAKE.JOBS.C=yes if this is supported,
and in that case jobs.mk will use JOB_MAX_C as default for JOB_MAX.

Remove logic from local.sys.mk which is now handled by jobs.mk

Also use latest dirdeps-targets.mk and meta.{autodep,subdir}.mk