add EADDRINUSE error code to sockets implementations
make WEBDAV_SERVER_QUIT_URL public:
send connection: close when server is stopping, unify connection close logic, make some structures in http_server.h private
fix bug during stopping server, occurs when all data has been queued up for the client already
Rename 'fdevent' module to 'event_loop', rename 'fd_t' to 'socket_t', remove 'http_backend' interface/implementations, move http/webdav server to use sockets/event_loop as a backend, extend http_server to shut down immediately even if there are open keep-alive clients connected

--HG--
rename : src/_fdevent_common.h => src/_event_loop_common.h
rename : src/fdevent.idef => src/event_loop.idef
rename : src/fdevent_select.c => src/event_loop_select.c
rename : src/fdevent_select.h => src/event_loop_select.h
rename : src/webdav_server_sockets_fs_main.c => src/webdav_server_fs_main.c
Fix windows and linux makefile configs to use EVENT_LOOP variables