mllib: add a binding for mkdtemp

It seems OCaml has no way to safely create a temporary directory, so add
a new binding to C's mkdtemp which does that.
builder: use a disposable GPG keyring for every Sigchecker

Create a temporary directory and tell gpg to use it as homedir, so
imported keys do not get into the user's keyring. This also avoid
importing the default key  when a different one is needed to check the
signature.

The only exception is when a non-default fingerprint is used: in this
case, that key is read from the user's keyring, since it is where it is.