libpthread: Implement sem_trywait() and sem_getvalue()

src/untrusted/pthread/nc_semaphore.c

Index: src/untrusted/pthread/nc_semaphore.c
diff --git a/src/untrusted/pthread/nc_semaphore.c b/src/untrusted/pthread/nc_semaphore.c
index 635dc859bc5c5fe4ca1b0504951001d6e9418e6d..cb1973c7a70ba14ca284f34375977a062a9e1ade 100644
--- a/src/untrusted/pthread/nc_semaphore.c
+++ b/src/untrusted/pthread/nc_semaphore.c
@@ -90,6 +90,14 @@ int sem_wait(sem_t *sem) {
   return 0;
 }
 
+int sem_trywait(sem_t *sem) {
+  if (decrement_if_positive(&sem->count))
+    return 0;
+
+  errno = EAGAIN;
+  return -1;
+}
+
 int sem_post(sem_t *sem) {
   /* Increment sem->count, checking for overflow. */
   int old_value;
@@ -114,3 +122,8 @@ int sem_post(sem_t *sem) {
   }
   return 0;
 }
+
+int sem_getvalue(sem_t *sem, int *value) {
+  *value = sem->count;
+  return 0;
+}