Make sure the server always replies to spy action queries.

When an error occurs a diplomat answer packet with the action type field set
to DIPLOMAT_ANY_ACTION is sent to let the client know that something went
wrong. The generic client code can then move on in the diplomat queue.

See bug #21646

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24515 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added individual client options to enable/disable sound effects, in-game music,
and menu music.

See patch #4493


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24516 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Added alpha_mod parameter to alphablit() of sdl2-client to get rid of
broken blend_surface() function.

See patch #4508


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24517 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Enable units that them self don't impose a ZOC but respect the ZOC of others

This is done by adding the HasNoZoc unit flag.

Requested by Edgaras Šeputis <morphles@gna>
Patch by Edgaras Šeputis <morphles@gna>
with the following modifications done by my self:
 * Add a separation line in is_my_zoc() (code style issue found by
   Marko Lindqvist <cazfi@gna>)
 * Align rule set documentation comments
 * Change "zones of control" to "zone of control"
 * generated help text: imposes (in stead of has) no zone of control" as
   Jacob Nevins <jtn@gna> suggested
 * generated help text: Specify that it imposes no zone of control on its
   adjacent tiles to avoid confusion between zone and tile.

See bug #21507

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24518 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Use choose_action_queue_next() when the intention is to move the diplomat queue along

Calling process_diplomat_arrival() with NULL as the first parameter moves
the diplomat queue along. It is not clear from its name or its parameters
that this is what will happen.

Define a new function choose_action_queue_next() that wraps
process_diplomat_arrival(). Use it when the intention is to move the
diplomat queue along.

See patch #4514

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24519 a0f10bec-cc02-0410-94fc-a9cfff90b4cd