Update dirdeps.mk et al to 20211212 versions

dirdeps.mk: simplify computation of qualified vs unqualified dirdeps.
Note the importance of avoiding unresolved variables in DIRDEPS
Fix DIRDEP_LOADAVG_REPORT - expr(1) fails if expression evaluates to 0
Trim ${SRCTOP}/ from debug etc messages to make comparison easier.
Include number of DIRDEPS in debug output.
Trim ${SRCTOP}/ when exporting to env while building DIRDEPS_CACHE
to help avoid env exhaustion.
Use DIRDEPS_ALL_MACHINES_FILTER to better handle ALL_MACHINES
in complex build environments.

dirdeps-options.mk: use separate .undef for each variable

dirdeps-targets.mk: allow for '.' in DIRDEPS_TARGETS_DIRS

meta.autodep.mk: leverage ${.SUFFIXES} if we can.

meta.sys.mk: allow use in META_MODE vs DIRDEPS_BUILD

meta2deps.py: compute a list of dirdep extensions that map to current
TARGET_SPEC to avoid confusion.

Reviewed by: stevek