util: use shm_open for in-memory files

shm_open is a POSIX function creating an in-memory file. Using it simplifies
the code and removes the dependency on XDG_RUNTIME_DIR. The only downside is
that we need to generate a random name for the shm file.

include/util/os-compatibility.h

@@ -1,9 +0,0 @@
-#ifndef UTIL_OS_COMPATIBILITY_H
-#define UTIL_OS_COMPATIBILITY_H
-
-int os_fd_set_cloexec(int fd);
-int set_cloexec_or_close(int fd);
-int create_tmpfile_cloexec(char *tmpname);
-int os_create_anonymous_file(off_t size);
-
-#endif

include/util/shm.h

@@ -0,0 +1,7 @@
+#ifndef UTIL_SHM_H
+#define UTIL_SHM_H
+
+int create_shm_file(void);
+int allocate_shm_file(size_t size);
+
+#endif