Build and install libtac as shared library

Additional install public headers for libtac shared library.
SO-Versioning is initially set to 0:0:0.
Fix Debian packaging due to README markdown rename

Regression introduced with ea841487
debian: Introduce shahred library packaging libtac
Introduce pkg-config file for libtac
Fix include directory in pkg-config file

All headers are stored in the libtac sub-directory.
Merge pull request #15 from grwlf/master

allow authorization without protocol defined
Merge pull request #13 from gollub/shared_libtac

[RFC] Build and install libtac as shared library
Merge pull request #14 from gollub/fix_debian_packaging

Fix Debian packaging due to README markdown rename
send 'cmd*' to provoke sending priv-lvl by the server

Normally, client sends 'exec' query to the server to receive 'priv-lvl'
attribute. In order to provoke sending default priv-lvl by the server at
autorizaton stage we send 'cmd*' attribute