Drop split-usr and unmerged-usr support

As previously announced, execute order 66:

https://lists.freedesktop.org/archives/systemd-devel/2022-September/048352.html

The meson options split-usr, rootlibdir and rootprefix become no-ops
that print a warning if they are set to anything other than the
default values. We can remove them in a future release.

src/basic/constants.h

@@ -59,22 +59,13 @@
 #define NOTIFY_FD_MAX 768
 #define NOTIFY_BUFFER_MAX PIPE_BUF
 
-#if HAVE_SPLIT_USR
-#  define _CONF_PATHS_SPLIT_USR_NULSTR(n) "/lib/" n "\0"
-#  define _CONF_PATHS_SPLIT_USR(n) , "/lib/" n
-#else
-#  define _CONF_PATHS_SPLIT_USR_NULSTR(n)
-#  define _CONF_PATHS_SPLIT_USR(n)
-#endif
-
 /* Return a nulstr for a standard cascade of configuration paths, suitable to pass to
  * conf_files_list_nulstr() to implement drop-in directories for extending configuration files. */
 #define CONF_PATHS_NULSTR(n)                    \
         "/etc/" n "\0"                          \
         "/run/" n "\0"                          \
         "/usr/local/lib/" n "\0"                \
-        "/usr/lib/" n "\0"                      \
-        _CONF_PATHS_SPLIT_USR_NULSTR(n)
+        "/usr/lib/" n "\0"
 
 #define CONF_PATHS_USR(n)                       \
         "/etc/" n,                              \
@@ -83,8 +74,7 @@
         "/usr/lib/" n
 
 #define CONF_PATHS(n)                           \
-        CONF_PATHS_USR(n)                       \
-        _CONF_PATHS_SPLIT_USR(n)
+        CONF_PATHS_USR(n)
 
 #define CONF_PATHS_USR_STRV(n)                  \
         STRV_MAKE(CONF_PATHS_USR(n))

src/basic/path-lookup.c

@@ -530,10 +530,6 @@ int lookup_paths_init(
         assert(scope >= 0);
         assert(scope < _RUNTIME_SCOPE_MAX);
 
-#if HAVE_SPLIT_USR
-        flags |= LOOKUP_PATHS_SPLIT_USR;
-#endif
-
         if (!empty_or_root(root_dir)) {
                 if (scope == RUNTIME_SCOPE_USER)
                         return -EINVAL;
@@ -625,6 +621,7 @@ int lookup_paths_init(
                                         "/usr/local/lib/systemd/system",
                                         SYSTEM_DATA_UNIT_DIR,
                                         "/usr/lib/systemd/system",
+                                        /* To be used ONLY for images which might be legacy split-usr */
                                         STRV_IFNOTNULL(flags & LOOKUP_PATHS_SPLIT_USR ? "/lib/systemd/system" : NULL),
                                         STRV_IFNOTNULL(generator_late));
                         break;

src/basic/path-lookup.h

@@ -10,7 +10,7 @@
 typedef enum LookupPathsFlags {
         LOOKUP_PATHS_EXCLUDE_GENERATED   = 1 << 0,
         LOOKUP_PATHS_TEMPORARY_GENERATED = 1 << 1,
-        LOOKUP_PATHS_SPLIT_USR           = 1 << 2,
+        LOOKUP_PATHS_SPLIT_USR           = 1 << 2, /* Legacy, use ONLY for image payloads which might be old */
 } LookupPathsFlags;
 
 typedef struct LookupPaths {

src/basic/path-util.h

@@ -25,20 +25,10 @@
 #  define PATH_SBIN_BIN_NULSTR(x) PATH_NORMAL_SBIN_BIN_NULSTR(x)
 #endif
 
-#define DEFAULT_PATH_NORMAL PATH_SBIN_BIN("/usr/local/") ":" PATH_SBIN_BIN("/usr/")
-#define DEFAULT_PATH_NORMAL_NULSTR PATH_SBIN_BIN_NULSTR("/usr/local/") PATH_SBIN_BIN_NULSTR("/usr/")
-#define DEFAULT_PATH_SPLIT_USR DEFAULT_PATH_NORMAL ":" PATH_SBIN_BIN("/")
-#define DEFAULT_PATH_SPLIT_USR_NULSTR DEFAULT_PATH_NORMAL_NULSTR PATH_SBIN_BIN_NULSTR("/")
+#define DEFAULT_PATH PATH_SBIN_BIN("/usr/local/") ":" PATH_SBIN_BIN("/usr/")
+#define DEFAULT_PATH_NULSTR PATH_SBIN_BIN_NULSTR("/usr/local/") PATH_SBIN_BIN_NULSTR("/usr/")
 #define DEFAULT_PATH_COMPAT PATH_SPLIT_SBIN_BIN("/usr/local/") ":" PATH_SPLIT_SBIN_BIN("/usr/") ":" PATH_SPLIT_SBIN_BIN("/")
 
-#if HAVE_SPLIT_USR
-#  define DEFAULT_PATH DEFAULT_PATH_SPLIT_USR
-#  define DEFAULT_PATH_NULSTR DEFAULT_PATH_SPLIT_USR_NULSTR
-#else
-#  define DEFAULT_PATH DEFAULT_PATH_NORMAL
-#  define DEFAULT_PATH_NULSTR DEFAULT_PATH_NORMAL_NULSTR
-#endif
-
 #ifndef DEFAULT_USER_PATH
 #  define DEFAULT_USER_PATH DEFAULT_PATH
 #endif