Mainline repo

[Torbjorn Granlund]

I am moving the nightly testing back from the gmp-5.0 repo to the
mainline gmp repo.  I'll keep the extra barbwire switched on for now; at
a later point I will have the nightly builds create two library builds,
one with extra checks and without.  The latter will be used for tuneup.

We made several bug fixes to gmp-5.0 that are not yet in the mainline
gmp.  Please remember to move your fixes over.  (It is probably best to
copy the change log entry into the same position, so that discrepances
therein better reflect actualy source code differences.  That helps me a
lot when making dot releases.)

-- 
Torbjörn