data: remove rsyslog config files

Many systems don't use rsyslog, others don't need seperate syslog files
for vdagent.

With systemd journal, It's possible to filter
spice-vdagent/spice-vdagentd log messages out of /var/log/messages:
journalctl SYSLOG_IDENTIFIER=spice-vdagent SLOG_IDENTIFIER=spice-vdagentd

This simplify spice-vdagent packaging and updates, since there are no
config files to deal with.

Related:
https://bugzilla.redhat.com/show_bug.cgi?id=1136881
Add g_return_if_fail() guards to file xfer public functions

With the next commit, we won't always have a file xfer object available.
This next commit will make sure NULL is never passed to the public file
xfer functions, but it's safer to guard against that anyway in case
this gets broken in the future.
Make creation of 'vdagent_file_xfers' optional

This commit only adds if (vdagent_file_xfers != NULL) {} checks before
calling into the file transfer code. This allows to disable file transfer
handling by setting this variable to NULL.
Disable file xfer when no suitable destination dir

Currently, if no suitable destination directory can be found, the file
transfer code defaults to using '.' as the destination directory. As this
is unlikely to work, better to just disable file transfers when we end up
in such a situation. This currently happens when spawning spice-vdagent
from gdm where we don't want file transfers to be available anyway.
Report an error when file transfers are disabled

This commit sends back a VD_AGENT_FILE_XFER_STATUS_ERROR to clients which
try to transfer files when the agent disabled file transfers (for example
because there is no valid destination directory for the transferred files).
vdagent-file-xfers: only open the file transfer dir when all the files were transferred

Once transferring multiple files is supported, do not bother the user
opening the file transfer directory for each file transferred, just do it
when the last file transfer is finished.