Created system call wrappers.c (and sbbswrap.h) and moved target platform-specific macros and code there.

Minor beautification by me

Exported freemail() function now prototyped.

No longer includes windows.h (included by client.h already)

The password is no longer checked against the user's current password when changing from the user defaults menu.

Change description of link to v3cfgfaq.txt.

Credits are only checked for downloads now (not getdate/size commands).