refactor: systemd units in services/, read with service_file like patches/

lib/common.sh

@@ -9,6 +9,17 @@ ok()    { echo -e "${c_green}[OK]${c_reset} $*"; }
 # patch_file <name>: a file from patches/ (module sources, patches). The
 # single-file build replaces this with one that has them embedded.
 patch_file() { cat "$SCRIPT_DIR/patches/$1"; }
+# service_file <name> [KEY=value...]: a systemd unit from services/, with
+# each @KEY@ replaced by its value. The single-file build replaces
+# service_raw like patch_file.
+service_raw() { cat "$SCRIPT_DIR/services/$1"; }
+service_file() {
+    local name="$1" kv
+    local -a subst=(-e '')
+    shift
+    for kv in "$@"; do subst+=(-e "s|@${kv%%=*}@|${kv#*=}|g"); done
+    service_raw "$name" | sed "${subst[@]}"
+}
 warn()  { echo -e "${c_yellow}[WARN]${c_reset} $*"; }
 err()   { echo -e "${c_red}[ERROR] $*${c_reset}" >&2; }
 ask_yn() {