fs: Fix relative directory creation incongruity with Boost.filesystem

The non-BFS version of create_directory_if_missing_recursive() handles
relative path names (e.g. "foo") correctly and doesn't attempt to create
a parent that is left unspecified in the path (i.e. empty), but the BFS
version does, predictably failing the whole operation.

This fixes an issue where `./wesnoth -p data foo` from the source tree
would fail with `error filesystem: Could not create parents to foo` if
`foo` did not already exist, when using the Boost.filesystem-based
implementation. `./wesnoth -p data ./foo` would succeed because there is
a visible parent directory '.' that already exists and needs not be
created again.