make t_immutable and 079 filesystem-agnostic

make t_immutable and 079 filesystem-agnostic

Changed the test 079 to be generic for all filesystems and to be executed
for all filesystems.  In src/t_immutable.c which is compiled for Linux
only, replace the old style XFS and ext2 specific code for setting the
append-only and immutable flags by generic code that makes use of the
ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_GETFLAGS) and remove the check for
the specific filesystem type.  FS_IOC_GETFLAGS/FS_IOC_SETFLAGS is always
used and placed in an '#ifdef FS_IOC_SETFLAGS' block in order to never fail
compilation.  Without support for FS_IOC_SETFLAGS, the test completes
with _notrun.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>