Update mkwinsyscall and make stand alone tool (#248)

* Add latest mkwinsyscall file

Copied file from:
https://github.com/golang/sys/blob/bc2c85ada10aa9b6aa9607e9ac9ad0761b95cf1d/windows/mkwinsyscall/mkwinsyscall.go

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

* Update to match hcsshim mksyscall_windows

Added hr error return type, cmdline flags for winio import, utf16, and
sorting.

Generated file has build constrain comment.

Input files can be specified with a glob pattern.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

* PR: update comment, use glob

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

tools/mkwinsyscall/doc.go

@@ -0,0 +1,52 @@
+/*
+mkwinsyscall generates windows system call bodies
+
+It parses all files specified on command line containing function
+prototypes (like syscall_windows.go) and prints system call bodies
+to standard output.
+
+The prototypes are marked by lines beginning with "//sys" and read
+like func declarations if //sys is replaced by func, but:
+
+  - The parameter lists must give a name for each argument. This
+    includes return parameters.
+
+  - The parameter lists must give a type for each argument:
+    the (x, y, z int) shorthand is not allowed.
+
+  - If the return parameter is an error number, it must be named err.
+
+  - If go func name needs to be different from its winapi dll name,
+    the winapi name could be specified at the end, after "=" sign, like
+    //sys LoadLibrary(libname string) (handle uint32, err error) = LoadLibraryA
+
+  - Each function that returns err needs to supply a condition, that
+    return value of winapi will be tested against to detect failure.
+    This would set err to windows "last-error", otherwise it will be nil.
+    The value can be provided at end of //sys declaration, like
+    //sys LoadLibrary(libname string) (handle uint32, err error) [failretval==-1] = LoadLibraryA
+    and is [failretval==0] by default.
+
+  - If the function name ends in a "?", then the function not existing is non-
+    fatal, and an error will be returned instead of panicking.
+
+Usage:
+
+    mkwinsyscall [flags] [path ...]
+
+The flags are:
+
+    -output string
+        Specify output file name (standard output if omitted).
+    -sort
+        Sort DLL and Function declarations (default true). Setting to false is intended to maintain
+		compatibility with older versions of mkwinsyscall so that diffs are easier to read and understand.
+    -systemdll
+        Specify that all DLLs should be loaded from the Windows system directory (default true).
+    -trace
+        Generate print statement after every syscall.
+    winio
+        Import this package ("github.com/Microsoft/go-winio").
+
+*/
+package main