terminal-util: beef up show_menu()

This modernizes the function a bit, and adds some bits:

1. whether to show numbers before entries is now optional, and if they
   are shown they are displayed in grey.

2. a common prefix can now be grayed out (later useful for completion
   support)

3. some variables have been named to clarify their purpose

4. the table display dimensions can now be auto-sized (by specifying
   SIZE_MAX and number of columns and column width)

src/basic/terminal-util.h

@@ -84,7 +84,7 @@ int read_one_char(FILE *f, char *ret, usec_t timeout, bool echo, bool *need_nl);
 int ask_char(char *ret, const char *replies, const char *text, ...) _printf_(3, 4);
 int ask_string(char **ret, const char *text, ...) _printf_(2, 3);
 bool any_key_to_proceed(void);
-int show_menu(char **x, unsigned n_columns, unsigned width, unsigned percentage);
+int show_menu(char **x, size_t n_columns, size_t column_width, unsigned ellipsize_percentage, const char *grey_prefix, bool with_numbers);
 
 int vt_disallocate(const char *name);