Switch "posix" to the new log channel registration mechanism

Summary:
This also removes magic rename code, which caused the channel to be
called "linux" when built on a linux machine, and "freebsd" when built
on a freebsd one, which seems unnecessary - registering a new channel is
sufficiently simple now that if we wish to log something extremely
os-specific, we can just create a new channel. None of the current
categories seem very specific to one OS or another.

Reviewers: emaste, krytarowski

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D30250

llvm-svn: 295954