asserts: add ifdbg macro, change dbg macro format

New macro ifdbg prints message when condition is true
asserts: put few asserts into connection.c

Mostly checking functions' arguments
asserts: put some asserts into event-loop.c

mostly 'sanity' asserts (check functions arguments etc.)
assert: put some asserts into wayland-client.c
asserts: include stdio in wayland-assert.h

to have all-inclusive ;)
asserts: some asserts in wayland-util.c
asserts: check if epoll_ctl succeded
asserts: check epoll_ctl's return value
asserts: added asserts

usually just "sanity" asserts (checking function arguments etc.)