REORG: cfgparse: move peers parsing to cfgparse-peers.c

REORG: cfgparse: move peers parsing to cfgparse-peers.c

This patch move the peers section parsing code from src/cfgparse.c to a
dedicated src/cfgparse-peers.c file. This seperation improves code
organization and prepares for further refactoring of the "peers" keyword
registration system.

No functional changes in this patch - the code is moved as-is with only
the necessary adjustments for compliation (adding SPDX header and
updating Makefile for build).

This is the first patch in a series to address issue #3221, which
reports that "peers" section keywords are not displayed with -dKall.