_mh_next_seq(): add 1 to the last used sequence number.

mh_append_message(): missed assignment to mhd member of struct _mh_data.
mh_message_number(): new function. It is the only way to get the message
sequence number.

@@ -34,7 +34,6 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <signal.h>
-#include <stdlib.h>
 #include <time.h>
 #include <string.h>
 #include <ctype.h>