Implement poll() on systems which support it properly.

Summary:
This eliminates the restriction on maximum socket descriptor number.

Partial backport of Core PR14336
https://github.com/bitcoin/bitcoin/pull/14336/commits/11cc491a288a73e911be24a285e12abd57df7d04

Depends on D6461

Test Plan:

ninja
ninja check-all
src/bitoind

Verify normal node behavior.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6470