refactor system initialization code

[zippy]

currently the base_defs.pl generates all the system definitions plus more, like the HTTP and TEST receptors/contexts.

We have to split this up so that we can spin up different versions of ceptr, i.e. ones without the test/shell contexts etc. and so we can move HTTP definitions into separate files.

• create a function that uses _t_parse to create a context and exectue the code in that context in a temporary instance of a receptor of that type so as to add definitions into it. I.e. this is a scaffold version of what's in base_defs.pl as a step on our way to a dev/compository protocol