node: Add chainman alias for g_chainman

Summary:
Partial backport (1/6) of core PR18698:
https://github.com/bitcoin/bitcoin/pull/18698/commits/fa7b626d7a150e5cbd4d163d2dab6f8a55fc2cc4#diff-d3c243938494b10666b44404a27af7d84b44a72b85a27431e0c89e181462ca6e

Depends on D8553.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8560