V2 initialization behavior

[david_abrahams]

Initialization
We check the name used to invoke Jam, and if the name 
is not the recognized Boost.Jam invocation we continue 
with the execution of the builtin Jambase. 

Otherwise, when we recognize a Boost.Jam invocation, 
we: 

We attempt to load "boost-build.jam" by searching from 
the current invocation directory up to the root of the 
file-system and in the paths specified by 
BOOST_BUILD_PATH. Within this, one invokes the "boost-
build" rule to indicate where the Boost.Build system 
files are, and to load them. 
If the Boost.Build system isn't loaded yet we error 
and exit, giving brief instructions on possible 
errors. 
We attempt to load "project-root.jam" by searching 
from the current invocation directory up to the root 
of the file-system. When found its location is noted 
as the PROJECT_ROOT. 
Finaly we load the "Jamfile" in the invocation 
directory. There are various spellings considered when 
finding the "Jamfile", the pattern for the name must 
fit: (J|j)amfile[.jam].