uid: add /etc/shadow and check_password function

uid: add /etc/shadow and check_password function

Implement UID Infrastructure: autenticação real (completed).

Added /etc/shadow file with basic entries (root:root, daemon:*, nobody:*).
Updated Makefile to include /etc/shadow in initrd.
Added check_password() function in pwd_grp.c to verify passwords against /etc/shadow.
Added check_password() declaration in pwd.h.
Password verification uses plaintext comparison for now (TODO: add SHA256/crypt).
Locked accounts (passwd starts with '*' or '!') are rejected.

Test: make test-battery PASS (157/157)