fix sem_open and sem_close to obey posix semantics

multiple opens of the same named semaphore must return the same
pointer, and only the last close can unmap it. thus the ugly global
state keeping track of mappings. the maximum number of distinct named
semaphores that can be opened is limited sufficiently small that the
linear searches take trivial time, especially compared to the syscall
overhead of these functions.

include/limits.h

@@ -30,6 +30,7 @@
 #define PTHREAD_STACK_MIN PAGE_SIZE
 #define PTHREAD_DESTRUCTOR_ITERATIONS 4
 #define SEM_VALUE_MAX 0x7fffffff
+#define SEM_NSEMS_MAX 256
 
 /* Arbitrary numbers... */