refactor: extract shared utility sources into src/bin/common/

[dimitri]

Summary

Move 16 .c/.h utility pairs out of src/bin/pg_autoctl/ into a new
src/bin/common/ directory, compiled into a static archive
libpgaf_common.a linked by pg_autoctl (and a future pgaftest
binary).

Motivation

The upcoming pgaftest test runner needs the same pgsql, pgsetup,
pgctl, parsing, ipaddr, and related utilities. Rather than copying
them, they live in one place and both binaries link the same archive.

Changes

New files

• src/bin/common/Makefile — builds libpgaf_common.a standalone
• src/bin/common/Makefile.common — shared build variables included
  by both pg_autoctl/Makefile and the upcoming pgaftest/Makefile;
  auto-detects macOS gettext path via brew --prefix gettext

Moved files (pg_autoctl/ → common/)

debian, env_utils, file_utils, ini_file, ini_implementation,
ipaddr, lock_utils, parsing, pgctl, pgsetup, pgsql,
pgtuning, pidfile, signals, string_utils, system_utils

Notable content improvements in the common/ copies

File	Change
pgsql.c/h	adds pgsql_alter_role_password() for the upcoming --autoctl-node-password feature
pgctl.c	adds --checkpoint=fast for pg_basebackup; fixes intToString dangling-pointer use
pgsetup.c, signals.c, ipaddr.c, ini_file.c	brace-style fixes from citus_indent

Updated Makefiles

• src/bin/Makefile — adds common target as a prerequisite for pg_autoctl
• src/bin/pg_autoctl/Makefile — includes Makefile.common and links $(COMMON_LIB) instead of compiling the moved sources directly

Testing

• Builds cleanly locally (make -C src/bin/)
• Style check passes: docker run --rm -v $(pwd):/repo -w /repo citus/stylechecker:no-py citus_indent --check