Inconsistent timeout unit in NetSocketPosix::poll

[MmAaXx500]

Godot version: Any

OS/device including version: Windows and Linux

Issue description:

Timeout unit is inconsistent in NetSocketPosix::poll.

For example if we input 1000 in p_timeout parameter.
On Windows we place this value in a timeval struct in this way:

godot/drivers/unix/net_socket_posix.cpp

Line 449 in 15a85fe

struct timeval timeout = { p_timeout, 0 };

witch means 1000 seconds and 0 microseconds.

On Linux we put the parameter directly into poll which takes the timeout in milliseconds:

godot/drivers/unix/net_socket_posix.cpp

Line 510 in 15a85fe

int ret = ::poll(&pfd, 1, p_timeout);

At the end we waited 1000 milliseconds on Linux and 1000 seconds on Windows which obviously not equal.

p.s.: I'm happily submitting a pr if we decided on the desired unit

Steps to reproduce: n/a

Minimal reproduction project: n/a

[Calinou]

cc @Faless

I think microseconds would be better here as they are more granular. Just make sure the documentation is up-to-date on this.