daemon_*.c: unit tests for code from src/ (for the code of cwdaemon).

tests_*.c: unit tests for code from tests/functional_tests/ (for the code of
functional tests, because I care about quality of functional tests too ;) ).

Tests from the first group are always enabled because compilation of cwdaemon
is always enabled.

Use "configure --enable-functional-tests" to enable unit tests from the
second group, because functional tests are disabled by default.

Use "make check" in top-level dir to run enabled tests.
