[SSSD] [PATCH] Add config_from_fd() to ini_config

[Stephen Gallagher]

The SSSD needs a config_from_fd() variant of the config_from_file() call
so that we can preopen a config file and perform some verification on it
before parsing it. The config_from_fd() call is used to avoid race
conditions between testing the file and reading it in.

Note: the *_from_fd() functions still require the config file name for
internal information. This does not imply that it is used to open the file.