Fix XNU / FreeBSD / OpenBSD / RHEL5 / NT bugs

For the first time ever, all tests in this codebase now pass, when
run automatically on macos, freebsd, openbsd, rhel5, rhel7, alpine
and windows via the network using the runit and runitd build tools

- Fix vfork exec path etc.
- Add XNU opendir() support
- Add OpenBSD opendir() support
- Add Linux history to syscalls.sh
- Use copy_file_range on FreeBSD 13+
- Fix system calls with 7+ arguments
- Fix Windows with greater than 16 FDs
- Fix RUNIT.COM and RUNITD.COM flakiness
- Fix OpenBSD munmap() when files are mapped
- Fix long double so it's actually long on Windows
- Fix OpenBSD truncate() and ftruncate() thunk typo
- Let Windows fcntl() be used on socket files descriptors
- Fix Windows fstat() which had an accidental printf statement
- Fix RHEL5 CLOCK_MONOTONIC by not aliasing to CLOCK_MONOTONIC_RAW

This is wonderful. I never could have dreamed it would be possible
to get it working so well on so many platforms with tiny binaries.

Fixes #31
Fixes #25
Fixes #14

libc/calls/hefty/vfork.S

@@ -22,28 +22,50 @@
 /	Forks process without copying page tables.
 /
 /	This is the same as fork() except it's optimized for the case
-/	where the caller invokes exec() immediately afterwards.
+/	where the caller invokes execve() immediately afterwards. You
+/	can also call functions like close(), dup2(), etc. You cannot
+/	call read() safely but you can call pread(). Call _exit() but
+/	don't call exit(). Look for the vforksafe function annotation
 /
 /	@return	pid of child process or 0 if forked process
 /	@returnstwice
-vfork:	testb	IsWindows()
+/	@vforksafe
+vfork:
+#if SupportsWindows()
+	testb	IsWindows()
 	jnz	fork$nt
+#endif
 	mov	__NR_vfork(%rip),%eax
-	cmp	$-1,%eax
-	je	systemfive.enosys
-	pop	%rsi
+	pop	%rsi			# saves return address in a register
+#if SupportsBsd()
 	testb	IsBsd()
 	jnz	vfork.bsd
+#endif
 	syscall
-	push	%rsi
-	cmp	$-4095,%rax
+	push	%rsi			# note it happens twice in same page
+	cmp	$-4095,%eax
 	jae	systemfive.error
-	ret
+0:	ezlea	__vforked,di
+	test	%eax,%eax
+	jz	1f
+	decl	(%rdi)
+	jns	2f			# openbsd doesn't actually share mem
+1:	incl	(%rdi)
+2:	ret
 	.endfn	vfork,globl
 
+#if SupportsBsd()
 vfork.bsd:
 	syscall
 	push	%rsi
 	jc	systemfive.errno
-	ret
+#if SupportsXnu()
+	testb	IsXnu()
+	jz	0b
+	neg	%edx			# edx is 0 for parent and 1 for child
+	not	%edx			# eax always returned with childs pid
+	and	%edx,%eax
+#endif /* XNU */
+	jmp	0b
 	.endfn	vfork.bsd
+#endif /* BSD */