implement POSIX timers

this implementation is superior to the glibc/nptl implementation, in
that it gives true realtime behavior. there is no risk of timer
expiration events being lost due to failed thread creation or failed
malloc, because the thread is created as time creation time, and
reused until the timer is deleted.

include/limits.h

@@ -31,6 +31,7 @@
 #define PTHREAD_DESTRUCTOR_ITERATIONS 4
 #define SEM_VALUE_MAX 0x7fffffff
 #define SEM_NSEMS_MAX 256
+#define DELAYTIMER_MAX 0x7fffffff
 
 /* Arbitrary numbers... */