libimcv: Support symlinks introduced by usrmerge

libimcv: Support symlinks introduced by usrmerge

Debian, Ubuntu, Fedora et. al. started to apply usrmerge to their
latest Linux distributions, i.e.  /bin, /sbin, and /lib are now
symbolical links to /usr/bin, /usr/sbin, and /usr/lib, respectively.
Since executables and libraries are contained only once in Linux
packages (e.g. /bin/cp in coreutils but not /usr/bin/cp) this leads
to missing file measurments due to the symlinks when doing remote
attestation.

The new ita_attr_symlinks PA-TNC attribute fixes this problem by
collecting symbolic links pointing to directories on the client
platform.