core: accept "|" ExecStart= prefix to spawn target user's shell; teach run0 about the new logic (#37071)

I've always been reluctant to invoke the current user's shell in another
user's context, hence was fully grounded in `sudo -i`. With this bit in
place `run0` will finally be feature-complete on my side ;-)

src/basic/env-util.c

@@ -885,9 +885,12 @@ int replace_env_argv(
                 char ***ret_bad_variables) {
 
         _cleanup_strv_free_ char **n = NULL, **unset_variables = NULL, **bad_variables = NULL;
-        size_t k = 0, l = 0;
+        size_t k = 0, l;
         int r;
 
+        assert(!strv_isempty(argv));
+        assert(ret);
+
         l = strv_length(argv);
 
         n = new(char*, l+1);

src/basic/path-util.h

@@ -145,6 +145,12 @@ static inline bool path_is_safe(const char *p) {
         return path_is_valid_full(p, /* accept_dot_dot= */ false);
 }
 bool path_is_normalized(const char *p) _pure_;
+static inline bool filename_or_absolute_path_is_valid(const char *p) {
+        if (path_is_absolute(p))
+                return path_is_valid(p);
+
+        return filename_is_valid(p);
+}
 
 int file_in_same_dir(const char *path, const char *filename, char **ret);