Update imsg*.[ch] from OpenBSD, add some compat bits it needs and remove some

bits it doesn't.

compat/asprintf.c

@@ -14,15 +14,14 @@
  * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#include <sys/types.h>
+
 #include <stdarg.h>
 #include <stdio.h>
-#ifdef HAVE_STDINT_H
-#include <stdint.h>
-#else
-#include <inttypes.h>
-#endif
 #include <string.h>
 
+#include "compat.h"
+
 int
 asprintf(char **ret, const char *fmt, ...)
 {