Make improvements

- Polyfill readlink("foo/") dir check on Windows
- Support asynchronous signal delivery on Windows
- Restore Windows Console from execve() daisy chain
- Work around bug in AARCH64 Optimized Routines memcmp()
- Disable unbourne.com shell completion on Windows for now
- Don't always set virtual terminal input state on console
- Remove Musl Libc's unusual preservation of realpath("//")
- Make realpath() strongly link malloc() to pass configure test
- Delete cosh.com shell, now that unbourne.com works on Windows!

libc/nt/kernel32/GetThreadContext.S

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