add upgrade command; reinstall keeps the same version

- terractl upgrade <name> lists the zips an instance can use; with a zip
  or 'latest' it checks the zip, stops the server, backs up worlds and
  configs to instances/.backups/, pins ZIP=, unpacks right away and
  starts the server again. Downgrades need --downgrade.
- reinstall pins the installed zip when ZIP= is blank, so it never
  changes version, and keeps server/.installed (uses a .reinstall flag).
- Resolve terractl's own path before cd /, so a relative $0 works.
- Docs, completion and the daemon's command list; version 0.0.12.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

completions/bash/terractl

@@ -4,16 +4,36 @@
 # Installed to <prefix>/share/bash-completion/completions/terractl, where the
 # bash-completion package loads it automatically. Also works with 'sudo terractl'.
 
-# Instances directory: default, or TERRARIA_BASE from /etc/default/terractl.
+# A setting from /etc/default/terractl, or its default: _terractl_setting <name> <default>.
 # The file is parsed rather than sourced, so nothing runs in your shell.
-_terractl_base() {
-    local base=/var/lib/terraria/instances value
+_terractl_setting() {
+    local result=$2 value
     if [[ -r /etc/default/terractl ]]; then
-        value=$(sed -n 's/^[[:space:]]*TERRARIA_BASE=//p' /etc/default/terractl | tail -n 1)
+        value=$(sed -n "s/^[[:space:]]*$1=//p" /etc/default/terractl | tail -n 1)
         value=${value%\"}; value=${value#\"}; value=${value%\'}; value=${value#\'}
-        if [[ -n $value ]]; then base=$value; fi
+        if [[ -n $value ]]; then result=$value; fi
     fi
-    printf '%s\n' "$base"
+    printf '%s\n' "$result"
+}
+
+# Instances directory
+_terractl_base() { _terractl_setting TERRARIA_BASE /var/lib/terraria/instances; }
+
+# Server zips an instance can use, by its TYPE in instance.conf (all zips if
+# that can't be read). On a remote, all of its zips, from its daemon.
+_terractl_zips() {
+    if [[ -n $2 ]]; then
+        timeout 5 terractl --host "$2" info 2>/dev/null </dev/null | sed -n 's/^dist_zip=//p'
+        return
+    fi
+    local dist type pattern='*.zip' f
+    dist=$(_terractl_setting TERRARIA_DIST /var/lib/terraria/dist)
+    type=$(sed -n 's/^TYPE=\([a-z]*\).*/\1/p' "$(_terractl_base)/$1/instance.conf" 2>/dev/null | tail -n 1)
+    case $type in
+        vanilla)    pattern='terraria-server-*.zip' ;;
+        tmodloader) pattern='tModLoader*.zip' ;;
+    esac
+    while IFS= read -r f; do printf '%s\n' "${f##*/}"; done < <(compgen -G "$dist/$pattern")
 }
 
 # Instance names. Reading the directory needs root or the 'terraria' group; if
@@ -105,7 +125,7 @@ _terractl() {
         cword=$COMP_CWORD
     fi
 
-    local commands="new list info start stop restart enable disable attach watch cmd mods reinstall fix-perms delete rename export import"
+    local commands="new list info start stop restart enable disable attach watch cmd mods upgrade reinstall fix-perms delete rename export import"
     local console="save say playing kick ban motd password time dawn noon dusk midnight settle seed version maxplayers port clear help"
 
     # terractl --host <remote> <command> ...: the same as on this host, except
@@ -173,6 +193,12 @@ _terractl() {
             if (( cword == 3 )); then _terractl_reply "$console"; fi ;;
         reinstall)
             if (( cword == 3 )); then _terractl_reply "--clean"; fi ;;
+        upgrade)
+            if [[ $cur == -* ]]; then
+                _terractl_reply "--downgrade --no-backup"
+            else
+                _terractl_reply "latest $(_terractl_zips "$instance" "$host")"
+            fi ;;
         enable|disable)
             if (( cword == 3 )); then _terractl_reply "--now"; fi ;;
         watch)