print_msg.sh: always call lpr in background

Prevent that the printer system lpr can block the Smuxi interface.
This can happen when the printer is stuck or there is no paper.

smuxi_hooks/on-message-received/print_msg.sh

@@ -11,15 +11,14 @@ if [ "$SMUXI_CHAT_ID" != "#dimsumlabs" ]; then
     exit 0
 fi
 
-# Sends ESC/P command
-send_esc() {
-	echo -en "\x1b$1" | lpr
-}
-
-echo -n "$SMUXI_MSG_TIMESTAMP_ISO_LOCAL " | lpr
-send_esc 4 # Select italic
-echo -n "$SMUXI_SENDER " | lpr
-send_esc 5 # Cancel italic
-echo $SMUXI_MSG \
-	| iconv -f utf-8 -t 437 -c \
-        | lpr &
+ESC="\x1b"
+ITALIC_ON="${ESC}4"
+ITALIC_OFF="${ESC}5"
+(
+	echo -n "$SMUXI_MSG_TIMESTAMP_ISO_LOCAL "
+	echo -ne "$ITALIC_ON"
+	echo -n "$SMUXI_SENDER"
+	echo -ne "$ITALIC_OFF"
+	echo " $SMUXI_MSG"
+) | iconv -f utf-8 -t 437 -c \
+  | lpr &