Add a new "pseudo" platform for POSIX. It it will contain code shared by the platforms supporting POSIX. Currently Linux, Mac OS and FreeBSD are considered POSIX platforms using the POSIX implementation.

Initially the platform socket implementation has been moved from the Linux, Mac OS and FreeBSD platforms to the POSIX platform.

If implementation which ends up in the POSIX platform at some point cannot be the same across POSIX platforms the implementation can easily be moved back into the individual POSIX "complient" platforms.
Review URL: http://codereview.chromium.org/51001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1583 ce2b1a6d-e550-0410-aec6-3dcde31c8c00