Configuration setting updates.

Allow use of environment variables (e.g. $HOME) in specs-file, resolved at
compiler run-time when processing the specs-file.  Added cachedir setting
where 'wget' (as a way of handling URLs) can dump stuff.
Added first attempt at http:// path handling.

The test program test_g15.gpp specifies inclusion of a file from
frmb.org, which it downloads into the cache-dir successfully.  Default
preferences are to allow copy-on-write for downloads and use the
cached copy as a preference (no re-download).  Some other fixes and
updates related to file-handling.
Started work on parallel-process handling.

Started on the handling for 'par' blocks;  these need to be abstracted
as C functions, so collect up free-variables (in FVNODE).