fd-util: add new acquire_data_fd() API helper

All this function does is place some data in an in-memory read-only fd,
that may be read back to get the original data back.

Doing this in a way that works everywhere, given the different kernels
we support as well as different privilege levels is surprisingly
complex.

src/basic/fd-util.h

@@ -77,6 +77,16 @@ int fd_get_path(int fd, char **ret);
 
 int move_fd(int from, int to, int cloexec);
 
+enum {
+        ACQUIRE_NO_DEV_NULL = 1 << 0,
+        ACQUIRE_NO_MEMFD    = 1 << 1,
+        ACQUIRE_NO_PIPE     = 1 << 2,
+        ACQUIRE_NO_TMPFILE  = 1 << 3,
+        ACQUIRE_NO_REGULAR  = 1 << 4,
+};
+
+int acquire_data_fd(const void *data, size_t size, unsigned flags);
+
 /* Hint: ENETUNREACH happens if we try to connect to "non-existing" special IP addresses, such as ::5 */
 #define ERRNO_IS_DISCONNECT(r) \
         IN_SET(r, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE, ENETUNREACH)