ospfd: compile warning cleanups

A set of patches to clarify some comments as well as cleanup code that was
causing warnings.  After these patches, the code can be compiled with

-Wall -Wsign-compare -Wpointer-arith -Wbad-function-cast -Wwrite-strings
-Wmissing-prototypes -Wmissing-declarations -Wchar-subscripts -Wcast-qual
-Wextra -Wno-unused-parameter -Wno-missing-field-initializers

(what is current in trunk plus -Wextra -Wno-unused-parameter
-Wno-missing-field-initializers).

Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
diff --git a/ospfd/ospfd.h b/ospfd/ospfd.h
index cc27f66..fb57bf5 100644
--- a/ospfd/ospfd.h
+++ b/ospfd/ospfd.h
@@ -273,7 +273,7 @@
   struct thread *t_write;
   struct thread *t_read;
   int fd;
-  int maxsndbuflen;
+  unsigned int maxsndbuflen;
   struct stream *ibuf;
   struct list *oi_write_q;