tree-wide: Introduce sd-forward.h and shared-forward.h headers

Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.

The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.

Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.

src/test/nss-test-util.h

@@ -3,7 +3,7 @@
 
 #include <nss.h>
 
-#include "forward.h"
+#include "shared-forward.h"
 
 const char* nss_status_to_string(enum nss_status status, char *buf, size_t buf_len);
 void* nss_open_handle(const char *dir, const char *module, int flags);

src/test/test-dlopen.c

@@ -3,7 +3,7 @@
 #include <dlfcn.h>
 #include <stdlib.h>
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int main(int argc, char **argv) {
         void *handles[argc - 1];

src/test/test-sizeof.c

@@ -12,7 +12,7 @@
 #define __STDC_WANT_IEC_60559_TYPES_EXT__
 #include <float.h>
 
-#include "forward.h"
+#include "shared-forward.h"
 
 /* Print information about various types. Useful when diagnosing
  * gcc diagnostics on an unfamiliar architecture. */