Move code related to configuration files in directories to new file

Move code related to configuration files in directories to new file

The code in charge of listing and classifying a set of configuration
files in a directory was located in guc-file.l, being used currently for
GUCs under "include_dir".  This code is planned to be used for an
upcoming feature able to include configuration files for ident and HBA
files from a directory, similarly to GUCs.  In both cases, the file
names, suffixed by ".conf", have to be ordered alphabetically.  This
logic is moved to a new file, called conffiles.c, so as it is easier to
share this facility between GUCs and the HBA/ident parsing logic.

Author: Julien Rouhaud, Michael Paquier
Discussion: https://postgr.es/m/Y2IgaH5YzIq2b+iR@paquier.xyz