unify logging of RS and VS

This fixes the bug of displaying a FWM service as [x.x.x.x]:0,
where x.x.x.x is the first RS of that service
unify connection options among checkers

All the remote checkers (TCP, HTTP/SSL, SMTP) now have the same set of connection options:
 * connect_ip (new to TCP, HTTP)
 * connect_port
 * bindto
 * bind_port  (new)
 * connect_timeout (new to SMTP)

All of them are optional with reasonable defaults.
The patch is designed for simplicity in adding a new option.

Since the connect_ip could be inequal to the RS address and, worse, the same for all RSes,
the endpoint is now logged as [RS]:rport, not the [connect_ip]:connect_port.
fwmark connection option
make SO_MARK a compile-time option
documentation for generic connection opts
random delay before doing the first check

every RS check is registered with a random delay
between 0 and vs->delay_loop seconds. It helps
avoiding multiple simultaneous checks to the same RS server.
Merge branch 'to_acassen' into ya_build_2