Restrict where INCREMENTAL.${NAME} files are recognized.

Restrict where INCREMENTAL.${NAME} files are recognized.

Previously, they were recognized anywhere in an incremental backup
directory; now, we restrict this to places where they are expected to
appear. That means this code will need updating if we ever do
incremental backups of files in other places (e.g. SLRU files), but
it lets you create a file called INCREMENTAL.config (or something like
that) at the top level of the data directory and still have things
work.

Patch by me, per request from David Steele, who also reviewed.

Discussion: http://postgr.es/m/5a7817da-6349-4653-8056-470300b6e512@pgmasters.net