Regression tests for system calls ported from NetBSD.

Reimplement ATF with many hacks to adjust the tests as little as possible.

Tests passing without source file adjustments:
t_access	t_getpid	t_link		t_msgsnd	t_sigaction
t_bind		t_getsid	t_listen	t_msync		t_socketpair
t_getgroups	t_getsockname	t_mkdir		t_pipe		t_truncate
t_getitimer	t_gettimeofday	t_mkfifo	t_sendrecv	t_umask
t_getlogin	t_kill		t_msgctl	t_setuid	t_write

Tests passing after adjustments:
t_chroot	- fchroot is not implemented
t_clock_gettime	- requires sysctlbyname
t_dup		- OpenBSD dup3 is similar to Linux dup3
t_fsync		- replace mkstemp
t_getrusage	- no expected fail, PR kern/30115 is NetBSD, work more
t_mknod 	- remove tests for unsupported file types
t_msgget	- remove msgget_limit test
t_poll 		- remove pollts_* tests
t_ptrace 	- change EPERM -> EINVAL for PT_ATTACH of a parent
t_revoke 	- remove basic tests, revoke only on ttys supported
t_select 	- remove sigset_t struct as it is int on OpenBSD

Failing tests:
t_mlock		- wrong errno, succeeds where not expected, POSIX imprecise
t_mmap		- ENOTBLK on test NetBSD is skipping, remove mmap_va0 test
t_msgrcv	- msgrcv(id, &r, 3 - 1, 0x41, 004000) != -1
t_pipe2 	- closefrom(4) == -1, remove F_GETNOSIGPIPE and nosigpipe test
t_stat 		- invalid GID with doas
t_syscall	- SIGSEGV
t_unlink	- wrong errno according to POSIX

Excluded tests:
t_clock_nanosleep	- not available
t_clone			- not available
t_connect		-
t_fork			-
t_getcontext		-
t_issetugid		-
t_kevent		-
t_lwp_create		- not available
t_lwp_ctl		- not available
t_mincore		- removed
t_minherit		-
t_mprotect		-
t_nanosleep		- not available
t_posix_fadvise		-
t_posix_fallocate	-
t_ptrace_wait		-
t_ptrace_wait3		-
t_ptrace_wait4		-
t_ptrace_wait6		- not implemented
t_ptrace_waitid		-
t_ptrace_waitpid	-
t_recvmmsg		-
t_sendmmsg		-
t_setrlimit		-
t_sigqueue		-
t_sigtimedwait		-
t_swapcontext		-
t_timer_create		-
t_ucontext		-
t_vfork			-
t_wait			-
t_wait_noproc		-
t_wait_noproc_wnohang	-