Introduce interprocess signaling on Windows

This change gets rsync working without any warning or errors. On Windows
we now create a bunch of C:\var\sig\x\y.pid shared memory files, so sigs
can be delivered between processes. WinMain() creates this file when the
process starts. If the program links signaling system calls then we make
a thread at startup too, which allows asynchronous delivery each quantum
and cancelation points can spot these signals potentially faster on wait

See #1240

libc/nt/files.h

@@ -225,6 +225,10 @@ bool32 GetVolumeInformationByHandle(int64_t hFile,
                                     char16_t *opt_out_lpFileSystemNameBuffer,
                                     uint32_t nFileSystemNameSize);
 
+uint32_t SetFilePointer(intptr_t hFile, int32_t lDistanceToMove,
+                        long *opt_inout_lpDistanceToMoveHigh,
+                        uint32_t dwMoveMethod);
+
 #if ShouldUseMsabiAttribute()
 #include "libc/nt/thunk/files.inc"
 #endif /* ShouldUseMsabiAttribute() */

libc/nt/kernel32/SetFilePointer.S

@@ -0,0 +1,18 @@
+#include "libc/nt/codegen.h"
+.imp	kernel32,__imp_SetFilePointer,SetFilePointer
+
+	.text.windows
+	.ftrace1
+SetFilePointer:
+	.ftrace2
+#ifdef __x86_64__
+	push	%rbp
+	mov	%rsp,%rbp
+	mov	__imp_SetFilePointer(%rip),%rax
+	jmp	__sysv2nt
+#elif defined(__aarch64__)
+	mov	x0,#0
+	ret
+#endif
+	.endfn	SetFilePointer,globl
+	.previous

libc/nt/master.sh

@@ -259,6 +259,7 @@ imp	'SetEvent'						SetEvent						kernel32	1
 imp	'SetFileAttributes'					SetFileAttributesW					kernel32	2
 imp	'SetFileCompletionNotificationModes'			SetFileCompletionNotificationModes			kernel32	2
 imp	'SetFileInformationByHandle'				SetFileInformationByHandle				kernel32	4
+imp	'SetFilePointer'					SetFilePointer						kernel32	4
 imp	'SetFileTime'						SetFileTime						kernel32	4
 imp	'SetFileValidData'					SetFileValidData					kernel32	2
 imp	'SetHandleCount'					SetHandleCount						kernel32	1