Add --system-identifier / -s option to pg_resetwal

hi hackers,

I was just involved in a DR case, where we needed to change system ID in
control data, and noticed that there is no such capability, officially – so
I thought, it would be good to have it

the attached patch adds a new -s / --system-identifier option to
pg_resetwal that allows users to change the database cluster's system
identifier; it can be useful when you need to make a restored cluster
distinct from the original, or when cloning for testing

some aspects about the patch:
- accepts positive 64-bit integers only (zero not allowed)
- requires interactive confirmation or --force flag for safety
- detects non-TTY environments and requires --force in scripts
- included input validation and error handling
- updated docs with clear warnings about compatibility
- added tests, including some edge cases

looking forward to hearing feedback!

Nik