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.
allow authorization without protocol defined

Protocol is only required for certain subset of services, mainly for ppp. We
allow authorization with empty protocol if user wants to use other service
names, like 'ssh'

From the http://tools.ietf.org/html/draft-grant-tacacs-02 page 30:

    The protocol attribute is intended for use with PPP. When service equals
    "ppp" and protocol equals "lcp", the message describes the PPP link
    layer service. For other values of protocol, this describes a PPP NCP
    (network layer service). A single PPP session can support multiple NCPs
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
Fix always true condition
The condition was checking tac_prompt which is a static array, so it
will always return some address. Now checking the value of the first
element of the array, which can be zero on non-zero.
C compiler hardening options
Add checks for C compiler hardening options (stack protector etc)
and use them if available.
Fix Debian build failing on AX_CHECK_COMPILE_FLAG
Whitespace fix
Merge pull request #16 from gollub/fix/debian_build_for_ax_check_compile_flag

Fix/debian build for ax check compile flag
Add source addr parameter for tac_connect_single

This allows to specify from which source address/interface the TACACS+
client connection gets initiated.

Bump SO-versioning due to API change.
Merge pull request #17 from gollub/feature/client_connect_source_address_v4

Add source addr parameter for tac_connect_single
Close /dev/urandom to fix open file handler leak.
Merge pull request #18 from pwdng/master

Close /dev/urandom to fix open file handler leak.
bump version to 1.3.9, mostly due to file descriptor leak fix
add Jeroen as primary author and list others from GitHub