utmp: add kernel utmp database infrastructure

utmp: add kernel utmp database infrastructure

Implement L2 (partial): kernel utmp database infrastructure.

Added utmp.h with Linux-compatible utmp structure and record types.
Implemented utmp.c with:
- utmp_init(): Initialize utmp database
- utmp_login(): Register user login session
- utmp_logout(): Mark session as DEAD_PROCESS
- utmp_dead(): Record process exit status
- utmp_get_by_pid(): Find utmp entry by PID

Reduced UTMP_MAX_ENTRIES to 32 to avoid BSS overflow.
Not initialized in boot (deferred to when needed) to prevent SMP panic.

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

Note: Userspace integration (getlogin/who) and login session
registration still needed for complete L2 implementation.