All programs in the (N)MH suite of programs use the
MHCONTEXT environment variable to find the so-called
context file where the current folder is remembered.
mhlib should do the same, so that it can be used in
combination with the standard (N)MH programs.

Also, when writing the context file, mhlib should
replace the Current-Folder line but keep the other
lines in tact.

The attached patch fixes both problems. It introduces a
new method for the class MH called getcontextfile which
uses the MHCONTEXT environment variable to find the
context file, and it uses the already existing function
updateline to update the context file.

Some questions concerning this patch:
- should I document the new method or should it be an
internal method only?
- should the fix be ported to older Python versions?
With the patch it does behave differently if you have
an MHCONTEXT environment variable.