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.

po/cachy-auto-update.pot

@@ -269,3 +269,69 @@ msgstr ""
 
 msgid "%d packages are being updated. Please leave the computer switched on until this is done."
 msgstr ""
+
+msgid "Settings"
+msgstr ""
+
+msgid "(none)"
+msgstr ""
+
+msgid "Up/Down select - Space or Right changes - q goes back"
+msgstr ""
+
+msgid "Package names separated by spaces, empty to clear:"
+msgstr ""
+
+msgid "Notify when an update starts"
+msgstr ""
+
+msgid "Notify after a successful update"
+msgstr ""
+
+msgid "Notify when something goes wrong"
+msgstr ""
+
+msgid "Notify when a restart is needed"
+msgstr ""
+
+msgid "Time between update runs"
+msgstr ""
+
+msgid "Postpone while a game is running"
+msgstr ""
+
+msgid "Only update on mains power"
+msgstr ""
+
+msgid "Minimum battery level (%)"
+msgstr ""
+
+msgid "Update AUR packages"
+msgstr ""
+
+msgid "Update Flatpaks"
+msgstr ""
+
+msgid "Update AppImages"
+msgstr ""
+
+msgid "Also rebuild -git packages"
+msgstr ""
+
+msgid "AUR helper"
+msgstr ""
+
+msgid "Resolve package conflicts automatically"
+msgstr ""
+
+msgid "Trim the package cache"
+msgstr ""
+
+msgid "Cached versions to keep"
+msgstr ""
+
+msgid "Remove packages nothing needs any more"
+msgstr ""
+
+msgid "Never update these packages"
+msgstr ""