known_dirs.c: Collapse GetInputDir(), GetMasterDir(), GetStateDir() with preprocessor
obliterate CFWORKDIR (GLOBAL_C) and its usage in the main codebase.

- CFWORKDIR globals remain in tests/{unit,load}/ but are unrelated
  to the main global that is being obliterated here.
examples/measure_log.cf: comment to account for potential compile-time statedir
RemoveDates(): move datetime string literal computation to compile-time

- comment on uncertainty of two-digit hh:mm:ss and time-zones shorter
  than three letters.

Pointed out by Edward Welbourne
libpromises/known_dirs.c: remove initializer from static array

No need to initialize the static arrary in GET_DEFAULT_DIRECTORY_DEFINE()
because statics are zero-filled at compile-time.
Changelog: update

Change: all embedded databases now rooted in state/ directory.
Feature: --with-statedir compile-time option to override state/ path.
tests/unit: convert unit tests from setenv to putenv usage

- tests/unit/db_concurrent_test.c
- tests/unit/db_test.c
- tests/unit/dbm_migration_bundles_test.c
- tests/unit/lastseen_migration_test.c
- tests/unit/lastseen_test.c
tests/load: convert load tests from setenv to putenv usage

- tests/load/db_load.c
- tests/load/lastseen_load.c
Remove unnecessary FILE_SEPARATOR when MapName() usage is imminent