ser-pipe.c porting to MinGW

[Andrew Cagney]

> The pipe_open() function written to work for solaris and linux uses
> sockets to re-direct the stdin and stdout of the child process.
> Unfortunately Microsoft do not allow sockets to be used with the posix
> compatibility functions read, write and dup2 under windows. 
> 
> Would the most sensible fix be to use standard pipes and change scb->fd
> to have scb->rfd and scd->wfd and then put a wrapper around the
> ser_unix_readchar and ser_unix_write functions in ser-pipe.c or can
> anyone suggest a more sensible fix. This will work on all hosts not just
> win32 so in should be a satisfactory solution, my only concern is
> renaming the scb->fd.

Ignoring mingw, given that pipe() is so much more portable then 
socket(), a rewrite, wouldn't hurt.

I just don't undersand your comment about ser_unix_*char.

Andrew