* glom/libglom/db_utils.[h|cc]: Added gda_cnc_string_encode() for use
with GdaConnection's cnc_string and auth_string, for instance to
escape the database directory, name, username, and password.
* glom/libglom/connectionpool_backends/postgres.cc:
* glom/libglom/connectionpool_backends/postgres_self.cc:
* glom/libglom/connectionpool_backends/sqlite.cc: Properly escape the
cnc_string and auth_string key values.

* tests/test_selfhosting_utils.cc: test_create_and_selfhost_from_example(),
test_create_and_selfhost_from_uri(): Add an optional subdirectory parameter
that we can use to force weird parts into the path.
* Makefile_tests.am:
* tests/test_selfhosting_new_from_example_strangepath.cc: Add a
test case that uses a path with some weird characters. This now works
thanks to the above changes.