Add a settings screen so nothing needs a text editor

All eighteen options are now reachable from the menu as a cursor list: arrows
select, Space or Right cycles a value, q goes back, changes are written
immediately. A numbered menu would have run out of digits.

Three real bugs surfaced while building it, each found by testing against an
actual pty rather than a pipe:

- Labels went through printf as format strings, so the percent sign in
  "Minimum battery level (%)" was an invalid conversion. cau_msg_in now only
  treats a message as a format string when arguments were actually passed -
  otherwise any literal % a translator writes is a trap.

- Mixing bash's line-mode read into a single-key interface left the following
  read -sn1 receiving nothing at all, reproducibly, so the screen froze after
  editing the package list. Replaced with a small line editor built on the same
  single-character reader.

- Backspace was being swallowed: in canonical mode DEL is the ERASE character
  and the line discipline consumes it, and bash returns to canonical mode
  between each read -sn1. The interface now holds non-canonical mode for its
  whole lifetime and hands the terminal back only around actions that print or
  prompt, with a trap restoring it on Ctrl-C.

Translations and config reads are memoized; the screen redraws every label on
every keypress and a fork per lookup was the reason the redraw was slow enough
to matter.

doc/cachy-auto-update.1.scd

@@ -16,7 +16,10 @@ updated in the background, with no password prompt and no terminal.
 
 Run without a command it opens a small interactive menu with the two switches
 that matter - automatic updates on/off and notifications on/off - plus the
-current status.
+current status, and a settings screen covering every remaining option so the
+configuration file never has to be edited by hand. In the settings screen the
+arrow keys select, Space or Right changes a value, and _q_ goes back; every
+change is written out immediately.
 
 The actual work is done by a systemd system service. The timer ticks hourly;
 whether a tick does anything is decided by _UpdateInterval_ (daily by default).