Lots of code refactoring, simplification and cleanup.
Lots of new unit-tests providing much higher code coverage.
All courtesy of rillig at netbsd.
Other significant changes:
o new read-only variable .SHELL which provides the path of the shell
used to run scripts (as defined by the .SHELL target).
o variable parsing detects more errors.
o new debug option -dl: LINT mode, does the equivalent of := for all
variable assignments so that file and line number are reported for
variable parse errors.
Changes since 20181221 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o meta.c: when target is out-of-date per normal make rules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
plus more unit-tests
