Update to bmake-20200517

Update to bmake-20200517

MFC of r341652, r342376, r361307

Changes since 20180512 are mostly portability related
hence the large gap in versions imported.

There are however some bug fixes, and a rework of filemon handling.
In NetBSD make/filemon/filemon_ktrace.c allows use of fktrace
and elimination of filemon(4) which has not had the TLC it needs.

FreeBSD filemon(4) is in much better shape, so bmake/filemon/filemon_dev.c
allows use of that, with a bit less overhead than the ktrace model.

Summary of changes from ChangeLog

	o str.c: empty string does not match % pattern
	  plus unit-test changes
	o var.c: import handling of old sysV style modifier using '%'
	o str.c: refactor brk_string
	o meta.c: meta_oodate, CHECK_VALID_META is too aggressive for CMD
	  a blank command is perfectly valid.
	o meta.c: meta_oodate, check for corrupted meta file
	  earlier and more often.
	* meta.c: meta_compat_parent check for USE_FILEMON
	  patch from Soeren Tempel
	o meta.c: fix compat mode, need to call meta_job_output()
	o job.c: extra fds for meta mode not needed if using filemon_dev
	o meta.c: avoid passing NULL to filemon_*() when meta_needed()
	  returns FALSE.
	o filemon/filemon_{dev,ktrace}.c: allow selection of
	  filemon implementation.  filemon_dev.c uses the kernel module
	  while filemon_ktrace.c leverages the fktrace api available in
	  NetBSD.  filemon_ktrace.c can hopefully form the basis for
	  adding support for other tracing mechanisms such as strace on
	  Linux.
	o meta.c: when target is out-of-date per normal make rules
	  record value of .OODATE in meta file.
	o parse.c: don't pass NULL to realpath(3)
	  some versions cannot handle it.
	o parse.c: ParseDoDependency: free paths rather than assert
	o parse.c: ParseVErrorInternal use .PARSEDIR
	  and apply if relative, and then use .PARSEFILE
	  for consistent result.
	o parse.c: ParseVErrorInternal use .CURDIR if .PARSEDIR
	  is relative
	o var.c: avoid SEGFAULT in .unexport-env
	  when MAKELEVEL is not set
	o var.c: add :q
	o dir.c: cleanup caching of stats

plus more unit-tests