add new 'file' function to be called when a file opens.
downgrade severity if functions don't load.

for expanding use cases of freeprocessors. also, simplify mem
handling and error messages when loading by getting rid of the
libname early; if it fails loading there's an ERR about it, we
don't need the libname in every message.
fix potential memleaks when loading processors.
Try to load ~/.situ/situ.cfg if there's no local file.
do not assume the user provides 'finish'.
allow interception of read-only opens.
remove situ.cfg in repo.
fclose: check for invalid function pointers.

with some notable binaries, e.g. 'ls', our constructor function
doesn't run on its own.  this causes pointers to still be null
when we intercept the corresponding functions, which leads us to
do terrible things.

grumble grumble.. but just call the initializer explicitly to work
around the problem.
add a tracker for provenance.