tmpfiles: add --tldr

This is like --cat-config, but omits the comments and empty lines.
The name is incoungrous with --cat-config, but I don't see a nice way to
call it that wouldn't be annoyingly long.

pager_open() is moved to cat_config() to remove some lines from run().

src/shared/pretty-print.h

@@ -15,8 +15,10 @@ int terminal_urlify_path(const char *path, const char *text, char **ret);
 int terminal_urlify_man(const char *page, const char *section, char **ret);
 
 typedef enum CatFlags {
-        CAT_FORMAT_HAS_SECTIONS = 1 << 0,  /* Sections are meaningful for this file format */
-        CAT_TLDR                = 1 << 1,  /* Only print comments and relevant section headers */
+        CAT_CONFIG_OFF          = 0,
+        CAT_CONFIG_ON           = 1 << 0,
+        CAT_FORMAT_HAS_SECTIONS = 1 << 1,  /* Sections are meaningful for this file format */
+        CAT_TLDR                = 1 << 2,  /* Only print comments and relevant section headers */
 } CatFlags;
 
 int cat_files(const char *file, char **dropins, CatFlags flags);