util/shm: add allocate_shm_file_pair

This function behaves like allocate_shm_file, except it also
returns a read-only FD. This is useful to share the same segment
of memory with many Wayland clients.

include/util/shm.h

@@ -1,7 +1,10 @@
 #ifndef UTIL_SHM_H
 #define UTIL_SHM_H
 
+#include <stdbool.h>
+
 int create_shm_file(void);
 int allocate_shm_file(size_t size);
+bool allocate_shm_file_pair(size_t size, int *rw_fd, int *ro_fd);
 
 #endif