* src/include/ecryptfs.h, src/libecryptfs/main.c,
    src/pam_ecryptfs/pam_ecryptfs.c, src/utils/Makefile.am,
    src/utils/mount.ecryptfs_private.c: move two functions from
    mount.ecryptfs_private to libecryptfs, namely is_mounted() and
    fetch_private_mnt(); use these in both pam_ecryptfs and
    mount.ecryptfs_private; also move PRIVATE to ECRYPTFS_PRIVATE in
    the ecryptfs.h headers; this will allow us to short-circuit some of the
    costly key-loading code on pam_auth if the private dir is already
    mounted, speeding up some subsequent authentications significantly,
    LP: #402748


Signed-off-by: Dustin Kirkland <kirkland@canonical.com>