Enable local llgs debugging on Linux when the use-llgs-for-local setting is enabled.

See http://reviews.llvm.org/D5695 for details.

This change does the following:

Enable lldb-gdbserver (llgs) usage for local-process Linux debugging.
To turn on local llgs debugging support, which is disabled by default, enable this setting:

(lldb) settings set platform.plugin.linux.use-llgs-for-local true
Adds a stream-based Dump() function to FileAction.
Pushes some platform methods that Linux (and FreeBSD) will want to share with MacOSX from PlatformDarwin into PlatformPOSIX.

Reviewed by Greg Clayton.