[PATCH] static analysis support for posix file desccriptor APIs

[Mir Immad]

 This is a patch for extending static analysis support for posix file
descriptor APIs which is a part of my GSoC project. I've written a few
testcases, which are all passing. There are a few TODOs like adding the
copyright header and adding docs to gcc/doc/invoke.texi.

I'm looking for suggestions for whether the names of warnings I have chosen
are appropriate, and configuring my vsocde editor to follow the GNU coding
conventions.

- Immad