Rename CHECKPOINT_FLUSH_ALL to CHECKPOINT_FLUSH_UNLOGGED.

Rename CHECKPOINT_FLUSH_ALL to CHECKPOINT_FLUSH_UNLOGGED.

The new name more accurately relects the effects of this flag on a
requested checkpoint.  Checkpoint-related log messages (i.e., those
controlled by the log_checkpoints configuration parameter) will now
say "flush-unlogged" instead of "flush-all", too.  This is
preparatory work for a follow-up commit that will add a
FLUSH_UNLOGGED option to the CHECKPOINT command.

Author: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de