BUG #1894: postmaster does not start

The following bug has been logged online:

Bug reference:      1894
Logged by:          Gerhard Kirchmann
Email address:      gerhard.kirchmann@arcor.de
PostgreSQL version: 8.1beta2
Operating system:   Linux x86
Description:        postmaster does not start
Details:

After installing 8.1.beta2 (and 8.1.beta1)
and after having performed an initdb (which produced no error)
starting postmaster always ends in the following error messages within the
file serverlog:
LOG:  could not bind IPv4 socket: Die Adresse wird bereits verwendet
HINT:  Is another postmaster already running on port 5432? If not, wait a
few seconds and retry.
FATAL:  database files are incompatible with server
DETAIL:  The database cluster was initialized with PG_CONTROL_VERSION 74,
but the server was compiled with PG_CONTROL_VERSION 811.
HINT:  It looks like you need to initdb.