lib: Rewrite FS type detection code + docs

Rewrite fs detection code to make it easily expandable.

Previously you had to add a function tst_is_cwd_foo() and yet another
if tst_is_cwd_foo() then tst_brkm() block into each testcase.

Now all you need is to add a fs magic constant into tst_fs_type.h and
case into a switch (tst_fs_type()) block into each testcase.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>