Handle reading of startup packet and authentication exchange after forking

Handle reading of startup packet and authentication exchange after forking
a new postmaster child process.  This should eliminate problems with
authentication blocking (e.g., ident, SSL init) and also reduce problems
with the accept queue filling up under heavy load.

The option to send elog output to a different file per backend (postgres -o)
has been disabled for now because the initialization would have to happen
in a different order and it's not clear we want to keep this anyway.