Move systemd_installation_has_version() to src/nspawn/

This function implements a heuristic that is only used by nspawn. It doesn't
belong in basic. I opted for a new file "nspawn-utils.c", because it seems
likely that we'll need some other new utilities like that in the future.

No functional change.

src/basic/path-util.h

@@ -181,8 +181,6 @@ bool is_device_path(const char *path);
 bool valid_device_node_path(const char *path);
 bool valid_device_allow_pattern(const char *path);
 
-int systemd_installation_has_version(const char *root, unsigned minimal_version);
-
 bool dot_or_dot_dot(const char *path);
 
 static inline const char *skip_dev_prefix(const char *p) {