confmodule: Align debconf's and cdebconf's confmodule

In preparation of extracting confmodule to its own module (see !11), these commits smooth out all differences between debconf's and cdebconf's confmodule.

There is a mirror MR for cdebconf: installer-team/cdebconf!19 (merged)

With both these MRs applied, cdebconf's confmodule becomes a strict backward-compatible superset of debconf's confmodule.

These are the remaining differences:

diff -U1 p c || true
--- p   2023-07-01 14:50:07.807623205 +0200
+++ c   2023-07-01 14:58:09.315956109 +0200
@@ -48,3 +48,3 @@
 
-# In cdebconf, the original stdin/stdout/stderr fds are duplicated onto fds
+# The original stdin/stdout/stderr fds are duplicated onto fds
 # DEBCONF_OLD_FD_BASE to DEBCONF_OLD_FD_BASE + 2. These values are exported
@@ -92,2 +92,3 @@
 db_endblock () { _db_cmd "ENDBLOCK" "$@"; }
+db_exist ()    { _db_cmd "EXIST" "$@"; }
 db_fget ()     { _db_cmd "FGET" "$@"; }
@@ -110,2 +111,3 @@
 db_x_loadtemplatefile ()       { _db_cmd "X_LOADTEMPLATEFILE" "$@"; }
+db_x_save ()   { _db_cmd "X_SAVE" "$@"; }