Convert configuration file parsing to use flex

Previously, repmgr was using a very simple ad-hoc string-based parser,
which had various limitations and allowed configuration files to be
created in a way which could cause confusion and/or unexpected
behaviour.

For example, it accepted strings enclosed in single quotes, but treated
strings enclosed in double quotes literally. A node_name defined thusly:

    node_name="somenode"

would result in the literal value '"somenode"' being used, which could
lead to unobvious errors along the lines of:

    no record found for ""somenode""

The configuration file parser has been adapted from the one used by
PostgreSQL itself, so behaves more-or-less identically (though some
functions such as file inclusion are not supported in repmgr).

This makes configuration parsing more robust and consistent;
additionally, error reporting will be more precise.

Note this does mean that some repmgr.conf items previously accepted
as valid by repmgr will now be rejected; in particular this includes
strings containing spaces which are not enclosed in single quotes.

.gitignore

@@ -53,3 +53,6 @@ repmgr
 repmgrd
 repmgr4
 repmgrd4
+
+# generated files
+configfile-scan.c