More minor bugfixes

- make pollcat_loop() work as expected with no structs linked, by
  - calling usleep() instead of poll()
  - refactoring delay calculation and reset into pollcat_time_value()
- simplify pollcat_timer_dispatch() in case of slow callbacks
  - deprecate pollcat_timer_dispatch_thresh
- revise tests/timer.c to support two new options:
  - -s to monitor stdin with pollcat_fd_add()
  - -w to use pollcat_loop() instead of handling main-loop separately
- make pollcat_dump() always present in header, since it's always compiled in
- add Makefile rules for installing tests, for easier running on cross-compiled targets

src/private.h

@@ -1,7 +1,7 @@
 /** Private includes
  *
  *  This file is part of the Pollcat Library.
- *  Copyright (C) 2022 Expatria Technologies Inc.
+ *  Copyright (C) 2022,2025 Expatria Technologies Inc.
  *  Contact: Morgan Hughes <morgan@expatria.ca>
  *
  *  The Pollcat Library is free software: you can redistribute it and/or modify it under
@@ -40,4 +40,8 @@
 extern pollcat_assert_f pollcat_assert_func;
 
 
+/** Return the current timeout and reset the current timeout to the base value */
+int pollcat_time_value (void);
+
+
 #endif /* _SRC_PRIVATE_H_ */