[rcore] Implement GetApplicationDirectory for FreeBSD

[dostoievsky]

• I tested it on latest raylib version from master branch
• I checked there is no similar issue already reported
• I checked the documentation on the wiki
• My code has no errors or misuse of raylib

Issue description

For loading resourcers it is needed to use GetApplicationDirectory function to change into resources directorie. On FreeBSD, there is no implemententation compatible, which makes applications unable to load files on other folders.

Environment

Provide your Platform, Operating System, OpenGL version, GPU details where you experienced the issue.

• FreeBSD
• I think the other details are irrelevant for this issue.

Issue Screenshot

Code Example

    const char* prev_dir = GetWorkingDirectory();
    const char* app_dir  = GetApplicationDirectory();
    ChangeDirectory(app_dir);
        Image fooImg = LoadImage("resources/foo.png");
    ChangeDirectory(prev_dir);