journalctl: add highlighting for matched substring

Red is used for highligting, the same as grep does. Except when the line is
highlighted red already, because it has high priority, in which case plain ansi
highlight is used for the matched substring.

Coloring is implemented for short and cat outputs, and not for other types.
I guess we could also add it for verbose output in the future.

src/basic/string-util.h

@@ -177,7 +177,7 @@ char* strshorten(char *s, size_t l);
 
 char *strreplace(const char *text, const char *old_string, const char *new_string);
 
-char *strip_tab_ansi(char **p, size_t *l);
+char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]);
 
 char *strextend_with_separator(char **x, const char *separator, ...) _sentinel_;