fix SYSCF so that things build when first defined

fix SYSCF so that things build when first defined

Things won't build for ports that first
define SYSCF.

This moves assure_syscf_file() from unixmain.c
to files.c and adjusts extern.h to get it
out from under #ifdef UNIX.

The call to assure_syscf_file() in options.c was
only #ifdef SYSCF, SYSCF_FILE and not UNIX,
so new ports #defining SYSCF would get an erro.

assure_syscf_file() will be utilized by mswin
when SYSCF is defined.