lxc-clone: don't s/oldname/newname in the config file and hooks

1. container hooks should use lxcpath and lxcname from the environment.
2. the utsname now gets separately updated
3. the rootfs path gets updated by the bdev backend.
4. the fstab mount targets should be relative
5. the fstab source directories could be separately updated if needed.

This leaves one definate bug: the lxc.logfile does not get updated.
This made me wonder why it was in the configuration file to begin with.
Digging deeper, I realized that whatever '-o outfile' you give
lxc-create gets set in log.c and gets used by the lxc_container object
we create at write_config().  So if you say
	lxc-create -t cirros -n c1 -o /tmp/out1
then /var/lib/lxc/c1/config will have lxc.logfile=/tmp/out1 - which is
clearly wrong.  Therefore I leave fixing that for later.

I'm looking for candidates for $p/$n expansion.  Note we can't expand
these at config_utsname() etc, because then lxc-clone would see the
expanded variable.  So we want to read $p/$n verbatim at config_*(),
and expand them only when they are used.  lxc.logfile is an obvious
good use case.  lxc.utsname can do it too, in case you want container
c1 to be called "c1-whatever".  I'm not sure that's worth it though.
Are there any others, or is that it?

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Logging: don't confuse command line and config file specified values

Currently if loglevel/logfile are specified on command line in a
program using LXC api, and that program does any
container->save_config(), then the new config will be saved with the
loglevel/logfile specified on command line.  This is wrong, especially
in the case of

cat > lxc.conf << EOF
lxc.logfile=a
EOF

lxc-create -t cirros -n c1 -o b

which will result in a container config with lxc.logfile=b.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>