Cluster name in ps output

Hi

When running more than one cluster I often find myself looking at
the output of 'iotop' or other tools wondering which
cluster's "wal receiver process" or "checkpointer process" etc
I'm seeing. Obviously it's easy enough to find out (for example
by looking at a tree view in htop/ps that shows the -D parameter
of the parent postmaster), but I thought it could be useful to be
able to give a name to the cluster and include it in the ps
output. Here is a strawman patch that does that.

If cluster_name is not set, it defaults to the empty string and
the ps output is unchanged. If it's set to 'foox' the ps output
includes that string in square brackets:

postgres: [foox] checkpointer process
postgres: [foox] writer process
postgres: [foox] wal writer process
postgres: [foox] autovacuum launcher process
postgres: [foox] stats collector process
postgres: [foox] munro foodb [local] idle

I would be grateful for any feedback. Thanks,

Thomas