load_main_config: use given path, store realpath
load_main_config: use given path, store realpath

Since `load_include_config` compares against the realpath of a config
file when checking if a config has already been added, the main config's
realpath has to be added to the config_chain.

However, includes from the main config should be processed relative to
the path given to allow for symbolic links. This stores the realpath in
`config->config_chain`, but uses the given path for all other
operations.

1ace/sway,ascent12/sway,ascent12/sway,SirCmpwn/sway,1ace/sway,1ace/sway,ascent12/sway