[Build] Enable SOUP session User Agent Customization

- Add the optional --with-ua-prefix flag to allow the customization
of the SOUP session User Agent Customization at build configuration time.

- Construct our user agent prefix with the following template:
    %s dLeyna/%VERSION

    %s is the value provided --with-ua-prefix if any
    %VERSION is the dLeyna version

- Call g_set_prgname(...) API to make our ua-prefix usable by libsoup.

Example:

with ./bootstrap-configure --with-ua-prefix=MypProject user agent value
will be:
      MyProject dLeyna/0.0.1 GUPnP/0.19.4 DLNADOC/1.50

if ./bootstrap-configure is called without --with-ua-prefix, user agent
value will be:
      dLeyna/0.0.1 GUPnP/0.19.4 DLNADOC/1.50

Fix issue: https://github.com/01org/renderer-service-upnp/issues/89

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>
[Documentation] Updated the README.txt file

Signed-off-by: Mark Ryan <mark.d.ryan@intel.com>