ser-unix.c: Add hardware flow control support

[Eli Zaretskii]

> Date: Mon, 21 May 2007 13:21:51 +0100 (BST)
> From: "Maciej W. Rozycki" <macro@mips.com>
> cc: gdb-patches@sourceware.org, Chris Dearman <chris@mips.com>,      "Maciej W. Rozycki" <macro@linux-mips.org>
> 
> gdb/doc/:
> 2007-05-21  Maciej W. Rozycki  <macro@mips.com>
> 
> 	* gdb.texinfo (Remote Configuration): Document "set/show 
> 	remoteflow".

This part is fine, but please add a @kindex index entry for the new
command, as we do in general for every other.