Fix spurious errors when writing to the child process stdin under Unix.

Since the child pipe was made non-blocking in r65993, it became possible to
write to child process without deadlocking when the pipe became full. However
this still resulted in an error from wxFileOutputStream as it didn't handle
EAGAIN returned from write() any differently than any other error, even though
it is an expected situation in this particular case.

Change Unix wxExecute() to use wxPipeOutputStream which ignores EAGAIN unlike
wxFileOutputStream to fix this.

See #12636.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66152 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/unix/pipe.h

@@ -118,6 +118,20 @@ public:
     virtual bool CanRead() const;
 };
 
+// ----------------------------------------------------------------------------
+// wxPipeOutputStream: stream for writing to a pipe
+// ----------------------------------------------------------------------------
+
+class wxPipeOutputStream : public wxFileOutputStream
+{
+public:
+    wxPipeOutputStream(int fd) : wxFileOutputStream(fd) { }
+
+    // Override the base class version to ignore "pipe full" errors: this is
+    // not an error for this class.
+    size_t OnSysWrite(const void *buffer, size_t size);
+};
+
 #endif // wxUSE_STREAMS && wxUSE_FILE
 
 #endif // _WX_UNIX_PIPE_H_