Turn off errexit option when detecting the apg command

Commit f32ac38c2f
allows to use pwgen to generate the initial password at the first
installation. However it won't work when apg isn't installed because:

* the postinst script has errexit option set ("set -e")
* at the first installation of ss-libev, apg or pwgen will be called to
  generate the initial password in config.json
* when apg isn't installed, "pathfind apg" would return 1
* and the postinst script will immediately exit due to "set -e"

This commit temporarily turn off the errexit option for pathfind()

debian/shadowsocks-libev.postinst

@@ -26,12 +26,14 @@ case "$1" in
 			cap_net_bind_service+ep /usr/bin/ss-server \
 			cap_net_bind_service+ep /usr/bin/ss-tunnel
 		if [ ! -f /etc/shadowsocks-libev/config.json ]; then
+			set +e
 			pathfind apg
 			if [ $? -eq 0 ]; then
 				passwd=$(apg -n 1 -M ncl)
 			else
 				passwd=$(pwgen 12 1)
 			fi
+			set -e
 			mkdir -p /etc/shadowsocks-libev
 			sed "s/barfoo!/$passwd/" /usr/share/shadowsocks-libev/config.json \
 				> /etc/shadowsocks-libev/config.json