fix: Use poll() for connect timeout on UNIX to avoid FD_SETSIZE limit

[tkarthikan]

Fixes #751

Problem
On UNIX systems, ConnectFDTimeout() uses select() to wait for a non-blocking connect to complete. However, select() has a hard limit of FD_SETSIZE (typically 1024) for file descriptors. When an application has many open connections and the socket fd is >= 1024, using FD_SET() causes undefined behavior, typically resulting in memory corruption or a crash.

Solution
Replace select() with poll() on UNIX platforms. poll() has no such fd limit since it uses an array of pollfd structures rather than a fixed-size bitmask.

Windows continues to use select() because its fd_set implementation is not bitmask-based and does not have this limitation.