msmtp: bump to version 1.6.0

In addition to bumping the version:

 - drop license comment from help, we have PKG_LICENSE* for that.
 - add optional dependency on libsecret
 - remove --without-gnome-keyring option

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>