tree-wide: add clickable man page link to all --help texts

This is a bit like the info link in most of GNU's --help texts, but we
don't do info but man pages, and we make them properly clickable on
terminal supporting that, because awesome.

I think it's generally advisable to link up our (brief) --help texts and
our (more comprehensive) man pages a bit, so this should be an easy and
straight-forward way to do it.

src/basic/terminal-util.c

@@ -1369,6 +1369,15 @@ int terminal_urlify_path(const char *path, const char *text, char **ret) {
         return terminal_urlify(url, text, ret);
 }
 
+int terminal_urlify_man(const char *page, const char *section, char **ret) {
+        const char *url, *text;
+
+        url = strjoina("man:", page, "(", section, ")");
+        text = strjoina(page, "(", section, ") man page");
+
+        return terminal_urlify(url, text, ret);
+}
+
 static int cat_file(const char *filename, bool newline) {
         _cleanup_fclose_ FILE *f = NULL;
         _cleanup_free_ char *urlified = NULL;

src/basic/terminal-util.h

@@ -157,6 +157,7 @@ int vt_reset_keyboard(int fd);
 
 int terminal_urlify(const char *url, const char *text, char **ret);
 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_FLAGS_MAIN_FILE_OPTIONAL = 1 << 0,