libkernel: setitimer is absent (PS5MC); branch ready on the alarm timer, never reporting an undelivered signal #3735

libkernel does not export setitimer (NID hPWDGx8ioXQ). PS5MC (PPSA99420) imports it (registry index 472, predicted FAIL: unresolved ELF import hPWDGx8ioXQ from libkernel.prx). The earlier #3293 was closed for reporting success without delivering the signal; this branch never does that.

Branch ready: screamm:feat/libkernel-setitimer (6bef7bf0, one commit on main a11c8f7f). I cannot open it as a PR because of the intake check; anyone eligible is welcome to carry it, keeping the commit as is. getitimer is left to the open #3572; whichever merges second must read the interval this branch stores (alarms.interval), since #3572 reports it as always 0.

• Built on the existing alarm timer thread, following FreeBSD 11.4 sys/kern/kern_time.c (sys_setitimer, kern_setitimer, realitexpire, itimerfix) and lib/libc/gen/alarm.c (8c639945): EINVAL for an unknown timer or a value/interval with a negative field, tv_usec >= 1000000 or more than INT32_MAX/2 seconds; a null value reads the timer (EFAULT when the output is null too, after the timer check as FreeBSD orders it); a zero value disarms and returns the previous remaining time and interval; a non-zero value arms, re-armed by it_interval on expiry; alarm now clears the interval as FreeBSD's does.
• What throws, with the reason named: expiry with a guest SIGALRM handler installed (no path runs a guest handler here; alarm already throws the same way), expiry while SIGALRM is blocked (pending signals are not modelled; this also corrects alarm, which wrongly exited in that case), an interval below 1 ms (FreeBSD rounds to tick/16 and the console's hz is unknown), and arming ITIMER_VIRTUAL/ITIMER_PROF (CPU-time accounting is not modelled; reading or disarming them is exact at zero). SIG_IGN drops the expiry and the default disposition exits with 142, as before.
• New guest_itimer (about 1 s) covers the error cases with the output untouched, disarm returning the previous value, the countdown, re-arm by interval, the CPU timers, the alarm interplay and the throw message at expiry (it_interval 0.250000 s) with the handler never running.

Tested on Windows 11 Pro 26220, WinLibs MinGW-w64 GCC 15.2.0 r7, Ninja, Release, BUILD_TESTING=ON, full build including libs: guest_itimer and guest_signals 3/3 (guest_itimer 10/10 in the worktree); ctest -j8 -E "^agc_driver_|^video_out_flip" 534/537, the failures being the known windows_exceptions (#3411, fixed by #3415), guest_kernel_sys (#3510) and guest_filesystem (#3518); with main'