global: add new function send_message

Let's continue in shortening the code. Send message will be
general function for send_client and send_display
server: use send_message in send_client

The send_client general functionality was moved to send_message so the
send_client is duplicated code.
global: check argument of CAN_CONTINUE op

argument can be either 0 or 1
server: remove unused variable
client: remove unused variables
client: add const modifier to add_listener

listener is const
tests: add parenthesis around an expression

in wit-test was assert evaluated in wrong order (| has lower precedence
that ==)
client: dispatch pending actions when freeing client
client: destroy compositor when freeing client

also fix codying style
client: replace send_display

Use new function kick_display (it just sends SIGUSR1 to display) and
send_message. I don't know why but passing vl_list from send_display
into send_message does not act correctly. (The new version of
send_display used send_message instead of switch). I've tried to debug it for
longer time now and it can ** /* cenzured */ ** me.
define event: swap const and static modifier

GCC says it should be so
server: replace send_client with send_message

The reason is the same as it was in client.c. The va_list acts
incorrectly.
server: check the argument

check if can_continue is 0 or 1
server: make wit_display_run return nothing

Since the wit_display_run can be interrupted and another action can take
place after it, returning exit status from it is useless.
global: fix types in assread and asswrite

I was comparing int to size_t
global: extend dbg messages

one extend, one add and remove unused variable. I know all three things
should have their own commit, but this change doesn't affect any
functionality so I decided to commit it at once
tests: suppress compiler warnings

Now these warrnings were from gcc (before from clang)
tests: make wit and proxy tests comply with wit_display_run prototype

wit_display_run doesn't return anything anymore
client: destroy object if it already exist when binding to registry

Or we will create a leak
client: uncomment seat_handle_name

In order not to get SIGSEGV