Abort pin monitoring on Linux for devices that don't support it

For some serial devices, such as ttyACM* USB devices, the ioctl(TIOCMIWAIT) returns an error. Now if an error occurs, we propogate it to the .NET implementation which already correctly ends the monitoring thread.

Previously, the error was ignored in a loop resulting in 100% CPU utilization.

Issue: DOTNET-85, #22

dll/serialunix/libnserial/modem.c

@@ -292,7 +292,13 @@ static serialmodemevent_t waitformodemevent(struct modemstate *mstate)
     risignal = -1;
 
   if (ioctl(mstate->handle->fd, TIOCMIWAIT, signals) < 0) {
+    if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR) {
+      return MODEMEVENT_NONE;
+    }
+
+    // Some USB drivers don't support modem signals.
     mstate->serialerror = ERRMSG_IOCTL;
+    mstate->posixerrno = errno;
     return -1;
   }
 
@@ -356,10 +362,17 @@ static void *modemeventthread(void *ptr)
 
   while (TRUE) {
     result = waitformodemevent(mstate);
-    if (result != MODEMEVENT_NONE && result != -1) {
+    if (result == -1) {
+      // We have a serious issue. We stop the thread. The error
+      // was already set by waitformodemevent.
+      mstate->eventresult = MODEMEVENT_NONE;
+      pthread_exit(NULL);
+    }
+    if (result != MODEMEVENT_NONE) {
       mstate->eventresult = result;
       pthread_exit(NULL);
     }
+    // else we just keep polling.
   }
 }
 #endif
@@ -403,9 +416,10 @@ NSERIAL_EXPORT serialmodemevent_t WINAPI serial_waitformodemevent(struct serialh
   handle->modemstate = &mstate;
   mstate.handle = handle;
   mstate.waitevent = event;
-  mstate.eventresult = 0;
   pthread_mutex_unlock(&(handle->modemmutex));
 
+  // We create the thread to wait on it afterwards, as we can
+  // make that thread cancellable.
   result = pthread_create(&(handle->modemthread), NULL,
 			  modemeventthread, &mstate);
   if (result == -1) {
@@ -421,6 +435,13 @@ NSERIAL_EXPORT serialmodemevent_t WINAPI serial_waitformodemevent(struct serialh
     return -1;
   }
 
+  if (mstate.serialerror != 0) {
+    errno = mstate.posixerrno;
+    serial_seterror(handle, mstate.serialerror);
+    handle->modemstate = NULL;
+    return -1;
+  }
+
   handle->modemstate = NULL;
   return mstate.eventresult & event;
 #else

dll/serialunix/libnserial/serialhandle.h

@@ -32,6 +32,7 @@ struct modemstate {
   serialmodemevent_t   waitevent;
   serialmodemevent_t   eventresult;
   int                  serialerror;
+  int                  posixerrno;
 };
 
 // Maximum number of ports we have in the port descrip