This option replaces --with-boost-process
This prepares external signer support to be disabled by default.
It adds a configure option to enable this feature and to check
if Boost::Process is present.
This also exposes ENABLE_EXTERNAL_SIGNER to the test suite via test/config.ini
The functional tests and util tests both require a config file that is
generated by ./configure. This commit merges those two config
files into a single configuration file that can be shared by both tests.
The config from config.ini is put into a Namespace object to maintain
the interface with bctest.py. A future commit could change this
interface to use a dictionary instead of a namespace.
