Refactor usage of PeekNamedPipe to get count rather than read

This makes it more obvious from reading the code that no characters
will be removed from the pipe as part of the implementation of
`can_receive`.

This commit includes renaming of local variables to exactly match the
names and types in the API definition, with nullptr used to distinguish
pointers from integers.

Author: thomasspriggs

src/util/piped_process.cpp

@@ -418,15 +418,22 @@ bool piped_processt::can_receive(optionalt<std::size_t> wait_time)
   const int timeout = wait_time ? narrow<int>(*wait_time) : -1;
 #ifdef _WIN32
   int waited_time = 0;
-  // The next four need to be initialised for compiler warnings
-  DWORD buffer = 0;
-  LPDWORD nbytes = 0;
-  LPDWORD rbytes = 0;
-  LPDWORD rmbytes = 0;
+  DWORD total_bytes_available = 0;
   while(timeout < 0 || waited_time >= timeout)
   {
-    PeekNamedPipe(child_std_OUT_Rd, &buffer, 1, nbytes, rbytes, rmbytes);
-    if(buffer != 0)
+    const LPVOID lpBuffer = nullptr;
+    const DWORD nBufferSize = 0;
+    const LPDWORD lpBytesRead = nullptr;
+    const LPDWORD lpTotalBytesAvail = &total_bytes_available;
+    const LPDWORD lpBytesLeftThisMessage = nullptr;
+    PeekNamedPipe(
+      child_std_OUT_Rd,
+      lpBuffer,
+      nBufferSize,
+      lpBytesRead,
+      lpTotalBytesAvail,
+      lpBytesLeftThisMessage);
+    if(total_bytes_available > 0)
     {
       return true;
     }