seeder: Use `poll()` instead of `select()`, if supported

Summary

This is a follow-up to !1386 (merged) in which the base net layer uses poll() over select() on certain platforms (currently just Linux).

However, the seeder is still using select(). select() suffers from some design problems in that if it happens to get given an FD > 1024 on Linux, it won't work. poll() suffers from no such limitation.

This commit takes the USE_POLL define, from compat.h, and if that is defined, and <poll.h> is also present at compile-time, then poll() will be used instead of select().

Also in this MR: tiny nits: prefer const, use size_t properly, explicit static_cast to int.

Test Plan

• ninja bitcoin-seeder check-bitcoin-seeder
• Also start up a seeder on Linux, and ensure it can communicate with nodes that it connects to. Perhaps uncomment the printfs in seeder/bitcoin.cpp lines 109-110 to verify it is communicating.
• (Opitional) Compile on a platform that doesn't have USE_POLL defined (such as BSD or OSX), or if you only use Linux, just undefine it yourself at the top of seeder/bitcoin.cpp, and ensure the seeder still operates correctly as before.