lib/tst_path_has_mnt_flags.c: create a function tst_path_has_mnt_flags()

Create a function tst_path_has_mnt_flags() for checking whether
a path is on a system that is mounted with specified flags.

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>