core: Add ExecSearchPath parameter to specify the directory relative to which binaries executed by Exec*= should be found

Currently there does not exist a way to specify a path relative to which
all binaries executed by Exec should be found. The only way is to
specify the absolute path.

This change implements the functionality to specify a path relative to which
binaries executed by Exec*= can be found.

Closes #6308

src/core/execute.h

@@ -254,6 +254,7 @@ struct ExecContext {
         char *smack_process_label;
 
         char **read_write_paths, **read_only_paths, **inaccessible_paths, **exec_paths, **no_exec_paths;
+        char **exec_search_path;
         unsigned long mount_flags;
         BindMount *bind_mounts;
         size_t n_bind_mounts;