move ptloader socket file into the shared socket directory

Summary:
In a Debian 10 default installation the folder for the ptloader socket
is missing /var/lib/imap/ptclient/ptsock. The folder gets created after the
first start by ptclient, but it's too late for the socket. After the 2nd restart
everything works fine as expected.

IMO it makes more sense to move the ptloader socket file into the shared socket
folder where all the other cyrus daemons have theirs:
/var/lib/imap/socket/ptsock.

This change only affects new installations.

Reviewers: PyKolab Developers, vanmeeuwen

Reviewed By: PyKolab Developers, vanmeeuwen

Subscribers: vanmeeuwen, balabadrasridhar212

Differential Revision: https://git.kolab.org/D889