[GRASS5] ANNOUNCE: GRASS 5.1 -> GRASS 5.7

[Markus Neteler]

GRASS 5.1 -> GRASS 5.7

Following the roadmap
   http://grass.itc.it/roadmap.html
I have made the (relevant) changes in CVS to rename
GRASS 5.1 to GRASS 5.7. Hopefully I caught all files.

The code compiles and works. To launch GRASS 5.7, run
   grass57
after compilation.

File names I left unchanges (e.g. grass51refman.dox)
so far. Maybe we should generally avoid version numbers
in file names in future whenever possible.

Cheers

 Markus