1) server.get_log(); server.seek_once(msg) -- since we get log object
2) server.logfile_pos.seek_once(msg) -- since the start of server

This patch also fixes incorrect offset calculation in buffer
with multiple responses:

    self.rbuf = string.sub(self.rbuf, off + 1)

`_func' space recovered before `_user' and auth_token can't
be retrived during creating struct func_def.

Extract the non-failing parts of the original patch by Roman.
Make an attempt at finding why coeio_custom is crashing when
replication_source is changed quickly - no success yet.

Fix some bugs in the function (now it uses libc's (3) dirname ffi.call).

A dropped user structure could still be accessed from sessions
which were authenticated with this user.

Check that a user is dropped when accessing it from the authenticated
session.

 + Add tarantoolctl enter instance_name command

This patch also changes box.info.server output for this case.

Other changes:

 - "" or NULL=ffi.cast('void *', 0) now resets option to default value
 - app/cfg.test.lua converted to TAP format