DESC
Move wrappers to an independant file and split the code more logically.
EDESC

Since I plan to add ANSI C wrappers too, it made more sense to split the
source files, leaving one for the core transaction API, another for common
functions and then one for each wrapper API implemented (unix.c and soon
ansi.c).

This patch does the split, adds a small document telling how it was done, and
modifies the Makefile accordingly.