IQA-463: Use APR file I/O for downloaded viewer installer .exe.

On Windows, calling CreateProcess(bInheritHandles=FALSE) is the wrong idea. In
that case, CreateProcess() passes NO handles -- even the files you've
explicitly designated as the child's stdin, stdout, stderr in the STARTUPINFO
struct! Remove LLProcess code to tweak bInheritHandles; we should also remove
the corresponding (useless) APR extension.
Instead, given that the Windows file-locking problem we've observed is
specific to the viewer installer .exe file downloaded by the background
updater logic, use APR file I/O for that specific file. Empirically, both
llofstream and std::ofstream seem to make the open file handle inheritable;
but apr_file_open() documentation says: "By default, the returned file
descriptor will not be inherited by child processes created by
apr_proc_create()." And indeed, it does appear to sidestep the locking problem.