Interning new keywords if specified with 'keyword:new-keyword' as in SBCL, CCL, Lispworks & Allegro

[alejandrozf]

I noticed that ABCL (as in ECL and CLISP) raises an error when one tries to specify a keyword using the package prefix

For example: keyword:world

In SBCL, CCL , Lispworks & Allegro CL this is allowed and a new keyword symbol is automatically interned. After reading the CL standard I'm not sure if this an issue solved in these implementations or just an extension implemented by them. But, in any case, I think is more comfortable for the user to allow it (even knowing that normally the package prefix is not used for specify keywords)

So, this PR implements this behavior for ABCL, interning automatically every new keyword symbol when using "keyword" as package prefix.