/Makefile: install: documented that schemas/public/install also tests that a clean public schema will be installable by full-database import

/Makefile: install: skip the test step at the end because this is not needed for the installation (it can be run separately if desired)

bugfix: **/postprocess.sql: don't use the public schema, because this creates an unsatisfied dependency while the database is being installed, and breaks `make install`

schemas/public_.sql: _km_to_m(): made it a wrapper around util._km_to_m() so this function is only in one place

schemas/util.sql: added _km_to_m() for use when public._km_to_m() is not defined yet

bugfix: benign_does_not_exist_error(): removed ignore_e=3, because this exit status is also used for other errors

fix: lib/sh/db.sh: benign_does_not_exist_error(): use benign_error=1, which is now supported properly by stderr_matches()

bugfix: lib/sh/util.sh: stderr_matches(): support $benign_error properly, by handling exit status logging in this func instead

bugfix: lib/sh/db.sh: pg_schema_exists(): also need to benignify "does not exist" error if returns false

bugfix: lib/sh/util.sh: stderr_matches(): need to separately display errors that were incorrectly suppressed due to $benign_error