Let var_* answer the optional add-on prompts (#17858)

Thirty-six yes/no prompts for optional components in 26 install
scripts could only be answered at the terminal, so an unattended
install stopped at the first one. Each now takes var_<app>_<option>
first and still asks when it is unset. The four conditions that only
matched a single y now take yes as well, since that is what the website
sends.

install/postgresql-install.sh

@@ -124,7 +124,8 @@ EOF
   systemctl restart postgresql
   msg_ok "Installed PostgreSQL"
 
-  read -r -p "${TAB3}Would you like to add Adminer? <y/N> " prompt
+  prompt="${var_postgresql_adminer:-}"
+  [[ -n "$prompt" ]] || read -r -p "${TAB3}Would you like to add Adminer? <y/N> " prompt
   if [[ "${prompt,,}" =~ ^(y|yes)$ ]]; then
     msg_info "Installing Adminer"
     $STD apt install -y adminer
@@ -158,7 +159,8 @@ setup_alpine() {
   $STD rc-service postgresql restart
   msg_ok "Configured and Restarted PostgreSQL"
 
-  read -r -p "${TAB3}Would you like to install Adminer with lighttpd? <y/N>: " prompt
+  prompt="${var_postgresql_adminer:-}"
+  [[ -n "$prompt" ]] || read -r -p "${TAB3}Would you like to install Adminer with lighttpd? <y/N>: " prompt
   if [[ ${prompt,,} =~ ^(y|yes)$ ]]; then
     msg_info "Installing Adminer and dependencies"
     $STD apk add --no-cache \