tree-wide: use path_implies_directory() instead of checking for a trailing slash

A path that ends in a slash must refer to a directory, but so must one
whose last component is "." or "..". We have path_implies_directory()
covering all these cases, hence use it where we so far just checked for
a trailing slash, to determine whether a path can only refer to a
directory.

This affects valid_shell() and valid_device_node_path() (no change in
behavior there, since the paths are refused anyway if not normalized),
and the check in the boot loader spec parser that refuses paths to
directories (where "/foo/." so far slipped through).

src/basic/path-util.c

@@ -1308,7 +1308,7 @@ bool valid_device_node_path(const char *path) {
         if (!PATH_STARTSWITH_SET(path, "/dev/", "/run/systemd/inaccessible/"))
                 return false;
 
-        if (endswith(path, "/")) /* can't be a device node if it ends in a slash */
+        if (path_implies_directory(path)) /* can't be a device node if it must be a directory */
                 return false;
 
         return path_is_normalized(path);

src/basic/user-util.c

@@ -915,7 +915,7 @@ bool valid_shell(const char *p) {
         if (!valid_home(p))
                 return false;
 
-        return !endswith(p, "/"); /* one additional restriction: shells may not be dirs */
+        return !path_implies_directory(p); /* one additional restriction: shells may not be dirs */
 }
 
 int maybe_setgroups(size_t size, const gid_t *list) {

src/shared/bootspec.c

@@ -172,8 +172,8 @@ static int mangle_path(
                 return -ENOMEM;
 
         /* We only reference files, never directories */
-        if (endswith(c, "/")) {
-                log_syntax(NULL, LOG_WARNING, fname, line, 0, "Path in field '%s' has trailing slash, ignoring: %s", field, c);
+        if (path_implies_directory(c)) {
+                log_syntax(NULL, LOG_WARNING, fname, line, 0, "Path in field '%s' refers to a directory, ignoring: %s", field, c);
                 *ret = NULL;
                 return 0;
         }