Meanwhile, database names with single quotes in names don't work very well

Meanwhile, database names with single quotes in names don't work very well
at all, and because of shell quoting rules this can't be fixed, so I put
in error messages to that end.

Also, calling create or drop database in a transaction block is not so
good either, because the file system mysteriously refuses to roll back rm
calls on transaction aborts. :) So I put in checks to see if a transaction
is in progress and signal an error.

Also I put the whole call in a transaction of its own to be able to roll
back changes to pg_database in case the file system operations fail.

The alternative location issues I posted recently were untouched, awaiting
the outcome of that discussion. Other than that, this should be much more
fool-proof now.

The docs I cleaned up as well.

Peter Eisentraut                  Sernanders väg 10:115