Attributes GH PR #924: https://github.com/ArcticaProject/nx-libs/pull/924

Run newer compilers in Ubuntu Bionic 18.04.

Also update gcc from v9.x to v10.x, and clang from v8.x to 9.x

Fix some syntax changes in TravisCI configuration.
Signed-off-by: Mario Trangoni <mjtrangoni@gmail.com>

Attributes GH PR #896: https://github.com/ArcticaProject/nx-libs/pull/896

In rootless mode some properties are private (or internal or
adminitrative). They are only required for the windows on the real X
server side that represent nxagent's windows. Those properties should
never be cloned from there to the the nxagent windows so we filter
them.

Fixes ArcticaProject/nx-libs#920

more output, correct format specifiers

reformat, extend comments, simplify ifdef

helps to avoid valgrind warnings

reformat, whitespace, code simplification, FIXMEs

... to match it as closely as possible to the upstream version.

The autograb stuff had been forgotten.

Fixes: ArcticaProject/nx-libs#635

This is a string that contains the address, the index, the PID and the
process name of the client. The string can be used in debugging
messages to identify the client.

The only place where this was referenced outside Client.c was a
commented section in Handlers.c. For now lets mask all that by an
(unset) define.

no functional change

Backport from this commit:

  commit 8dfd98245d2c44a1eb4c8b7c275e6cfc10fe40f1
  Author: Chase Douglas <chase.douglas@canonical.com>
  Date:   Wed Apr 11 11:14:51 2012 -0700

    Fix copy/paste error from before git history in UpdateCurrentTimeIf()

    See UpdateCurrentTime() for reference. I don't know what bug this might
    trigger, but it wouldn't hurt to fix this.
Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

Looks like a copy'n'paste error.

to make them all fit in the same naming scheme that better describes their purpose.

drop the nxagent prefix to emphasize they are not called from the
outside. Also make them static and add prototypes, if missing.