Windows and Linux allow the storage of configuration files relative
to the binary location, which is enabled by default on Windows and has
to be explicitly enabled on Linux.
This was originally conflated with the LINUX_PORTABLE build setting
which also allowed the application itself to be run from non-default
locations on a Linux system.
This change reintroduces the functionality behind the
ENABLE_PORTABLE_CONFIG build setting on Linux.
It also adds necessary code to make this setting compatible with the
recently introduced relocatable settings code changes:
When portable mode is enabled, user configuration, scene collections,
and profiles are stored in the config directory created for portable
mode.
