uid: add login command with password authentication

uid: add login command with password authentication

Implement UID Infrastructure: login real (completed).

Added user/cmds/login/login.c with:
- Username prompt
- Password prompt with echo disabled (termios)
- Password verification using check_password()
- UID/GID setting after successful authentication
- Home directory change
- Shell execution (from /etc/passwd or default /bin/sh)
- Loop for retry on failed login

Added user/cmds/login/Makefile.
Updated Makefile to include login in USER_CMD_NAMES.

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