Resolve "Tests fail when symlink support is disabled"

This MR does several things:

1. SUPPORT_SYMLINKS is now disabled by default. It didn't do much in the first place and only affects incomplete code. The corresponding README entry has been removed. The only thing it does support is accessing GekkoFS from a foreign namespace via a symbolic link. However, only stat seems to be working.
2. GKFS_FOLLOW_EXTERNAL_SYMLINKS was also disabled by default in this MR. This caused the test_symlink to fail as it tested external symlinks into GekkoFS, not actual symlinks within GekkoFS. This can only be done via lstat() for each component of the path which is a performance risk under certain circumstances. Overall, this is the relevant CMake variable for the test.
3. Unify code formatting for CMake files.

Closes #298 (closed)