util: New function ni_file_exists_fmt()

It can be used to create a concatenated pathname and
check for existance of a file.
client: Changing ifconfig reading and loading

New file: read-config.c

New function ni_ifconfig_read() added as well as
multiple other read() function for different type of
config source.
config: Change config sources handling logic
client: Add show-config parameter

show-config displays configs (pre-parsed to xml)
from all types of sources.