Hiding the child process window is not strictly necessary but it avoids opening
an empty shell window when running hg serve as well as a task in the task bar.
The window is hidden after the process is already started causing a single
flicker.
This fixes the implied reliance on pywin32 and the win32 module. This
also fixes a regression in fe7c89838a64 that made Mercurial unusable
without pywin32.
The posixfile_nt class has been superseded by posixfile in osutils.c,
which works on Windows NT and above. All other systems get the regular
python file class which is assigned to posixfile in posix.py (for POSIX)
and in the pure python version of osutils.py (for Win 9x or Windows NT
in pure mode).