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

include/pollcat/timer.h

@@ -28,8 +28,9 @@
 typedef uint64_t pollcat_time_t;
 
 
-/** Grace period in milliseconds for late timers, set to 0 for exact match */
-extern pollcat_time_t  pollcat_timer_dispatch_thresh;
+/** This was removed and replaced with a simpler implementation which allowed slow/late
+ *  timers to run reliably.  Setting it now has no effect. */
+extern pollcat_time_t  pollcat_timer_dispatch_thresh __attribute__((deprecated));
 
 
 /** Maximum number of times to run the dispatch loop, set to -1 for no limit */