XSB Src (SVN)-old

Added xsb_throw_error to allow more precise error messages than
obtained by the very general xsb_abort. Used it in biassert to better
categorize assert error messages. Perhaps other uses of xsb_abort
might be changed.

Exported xsb_sprint_variable so C programs can use it. (Asked by by MK).

Removed default assumption in setting CHARACTER_SET, now returning
error if unrecognized char-set.

Fixed bug introduced in returing strings, when added character sets.
Also added new better error reporting.

Minor change to fix a compiler warning on the mac.

Forgot to commit earlier.

Previous commit had bug. Set ep of old usermod psc record only if it
is undefined.

Changed slightly handling of exported predicates when loading a module.
The new psc record for an exported predicate has, for a long time, been
added to usermod, so that after consulting a module, the user can directly
call it from the top-level. But if there previously was a psc record
for the same-named predicate already in usermod, it would be lost, and so
calls to "that" psc record will not be found. So I changed it so that
old usermod psc record now branches to the ep of the newly added psc record
so it works as expected. I used the import_as mechanism to get this done.

Forgot to add.

Added documentation of new library prolog_db.