fix bug #1977: remove etags from database after version rollback so that the client can detect the changes

add parent directory to the file cache if it does not exist yet.

fix for bug #1942: Allow user to share with every group if "allow users to share with anyone" is set

TODO for OC5: use CURRENT_TIMESTAMP for all databases. mysql supports it as a default for DATETIME since 5.6.5 [1]
as a fallback we could use <default>0000-01-01 00:00:00</default> everywhere
[1] http://bugs.mysql.com/bug.php?id=27645
http://dev.mysql.com/doc/refman/5.0/en/timestamp-initialization.html
http://www.postgresql.org/docs/8.1/static/functions-datetime.html
http://www.sqlite.org/lang_createtable.html
http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions037.htm

put all the calls into one function and use tabs instead of spaces. Thanks to danimo for bugging me :-)