Refactor CREATE/ALTER DATABASE syntax so options need not be keywords.

Most of the existing option names are keywords anyway, but we can get rid
of LC_COLLATE and LC_CTYPE as keywords known to the lexer/grammar.  This
immediately reduces the size of the grammar tables by about 8KB, and will
save more when we add additional CREATE/ALTER DATABASE options in future.

A side effect of the implementation is that the CONNECTION LIMIT option
can now also be spelled CONNECTION_LIMIT.  We choose not to document this,
however.

Vik Fearing, based on a suggestion by me; reviewed by Pavel Stehule