add isastream (obsolete STREAMS junk)

apparently some packages see stropts.h and want to be able to use
this. the implementation checks that the file descriptor is valid by
using fcntl/F_GETFD so it can report an error if not (as specified).

src/misc/isastream.c

@@ -0,0 +1,7 @@
+#include <stropts.h>
+#include <fcntl.h>
+
+int isastream(int fd)
+{
+	return fcntl(fd, F_GETFD) < 0 ? -1 : 0;
+}