expand env variable in preconfigure seafile directory
expand env variable in preconfigure seafile directory

Closes #578.

daodaoliang/seafile-client,wgaalves/seafile-client,haiwen/seafile-client,losingle/seafile-client,losingle/seafile-client,losingle/seafile-client,lucius-feng/seafile-client,wgaalves/seafile-client,haiwen/seafile-client,wgaalves/seafile-client,lucius-feng/seafile-client,daodaoliang/seafile-client,daodaoliang/seafile-client,haiwen/seafile-client,lucius-feng/seafile-client,wgaalves/seafile-client,losingle/seafile-client,daodaoliang/seafile-client,lucius-feng/seafile-client,haiwen/seafile-client