BUG #3888: postmaster: misleading error message for illegal -B values

The following bug has been logged online:

Bug reference:      3888
Logged by:          Marti Raudsepp
Email address:      marti@juffo.org
PostgreSQL version: 8.3RC1
Operating system:   Linux AMD64
Description:        postmaster: misleading error message for illegal -B
values
Details:

When launching postmaster with a small -B (number of buffers) value, it
reports the following error:
"postmaster: the number of buffers (-B) must be at least twice the number of
allowed connections (-N) and at least 16"

However, as of PostgreSQL 8.3, postmaster requires that the number of
buffers is at least (max_connections*2 + 6)

To reproduce the bug:
# sudo -u postgres postmaster -D $PGDATA -N 8 -B 16
postmaster: the number of buffers (-B) must be at least twice the number of
allowed connections (-N) and at least 16
# sudo -u postgres postmaster -D $PGDATA -N 8 -B 21
postmaster: the number of buffers (-B) must be at least twice the number of
allowed connections (-N) and at least 16

Specifying -B 22 works as expected.