autotools: Avoid multiple liblxc.so with --enable-pam

autotools: Avoid multiple liblxc.so with --enable-pam

When installing LXC with the default options, a single non-symlink liblxc.so*
(e.g. liblxc.so.1.7.0) file is created:

```
    $ ./autogen.sh && ./configure && make && \
    rm -rf "$HOME/lxci" && make DESTDIR="$HOME/lxci" install && \
    stat -c%N "$HOME/lxci/usr/local/lib/liblxc.so"*
    [...]
    '/home/someone/lxci/usr/local/lib/liblxc.so' -> 'liblxc.so.1'
    '/home/someone/lxci/usr/local/lib/liblxc.so.1' -> 'liblxc.so.1.7.0'
    '/home/someone/lxci/usr/local/lib/liblxc.so.1.7.0'
```

However, when automake>=1.16.5, and the `--enable-pam` option is used, two
non-symlink liblxc.so* (e.g. liblxc.so.1.0.0 and liblxc.so.1.7.0) are
erroneously created:

```
    $ ./autogen.sh && ./configure --enable-pam && make && \
    rm -rf "$HOME/lxci" && make DESTDIR="$HOME/lxci" install && \
    stat -c%N "$HOME/lxci/usr/local/lib/liblxc.so"*
    [...]
    '/home/someone/lxci/usr/local/lib/liblxc.so' -> 'liblxc.so.1.0.0'
    '/home/someone/lxci/usr/local/lib/liblxc.so.1' -> 'liblxc.so.1.0.0'
    '/home/someone/lxci/usr/local/lib/liblxc.so.1.0.0'
    '/home/someone/lxci/usr/local/lib/liblxc.so.1.7.0'
```

This is due to infighting between libtool's and LXC's versioning:
libtool creates liblxc.so.1.0.0, then LXC's `install-exec-local` hook in
`Makefile.am` moves it to liblxc.so.1.7.0. However, with `--enable-pam`, the
`install-libLTLIBRARIES` target is re-triggered after `install-pamLTLIBRARIES`,
which will create liblxc.so.1.0.0 again.

The bigger problem here is that the install for the pam_cgfs library is done on
the `data` phase of the automake install process instead of the `exec` phase
(https://www.gnu.org/software/automake/manual/html_node/The-Two-Parts-of-Install.html),
which gives `install-libLTLIBRARIES` a chance to run again after the
`install-exec-local` / `install-exec-hook` targets have already run.

To fix this, we add an "exec_" prefix to the pam_cgfs library to make it run
during the `exec` phase (see link above). We also consolidate the various hooks
in the `install-exec-hook` target, which runs after the whole install, avoiding
needing to manually specify the dependencies like in `install-exec-local`.

Signed-off-by: Joan Bruguera <joanbrugueram@gmail.com>