[gforth] Some little changes I did to support Gforth 0.7.9

[Marcos Cruz]

The migration from Gforth 0.7.3 to 0.7.9 broke most of my programs, but
it was easy to solve.

For example, many of my programs used the undocumented string operator
`s+`, which, as the repository shows, was removed years ago. I simply
added it to my personal library.

Also, in Gforth 0.7.3 `basename`, `dirname`, and `scan-back` were
defined in `forth-wordlist`, but in Gforth 0.7.9 they are defined in the
`c-lib` vocabulary. I added them to my personal library, simply as
synonyms in `forth-wordlist`, to make them accessible as before.

No problem. Just wanted to share.

-- 
Marcos Cruz
http://programandala.net