sf_db: Use new temp file mechanism.
sf_db: Readonly functions can now get DB content from stdin

This is used by new sf_cfg functions, as default values are spread among several files, but we want to manage them as if they were in a single DB.
In order to have functions read from stdin, set $SF_DB_PATH to '<stdin>'.
Add sf_cfg module

This module allows to manage configuration parameters. The main difference with plain DB entries is that the list of parameters is known and each parameter has a default value. External software define their parameters by adding a file to a given directory. This file contains the list of parameters and default values.
Set version to 1.27.0
Merge branch 'release-1.27.0' into develop