Add FORTIFY and SANITIZE options for DEBUG builds

SANITIZE is not fully supported (or recommended) yet.

FORTIFY will include some performance hits, but those might be offset by -O1
(which is apparently required for _FORTIFY_SOURCE to have any effect).
Prevviosly, DEBUG builds used (implicit -O0).
I'm building and running sbbs on Vertrauen (git.synchro.net) now with FORTIFY.

Set FORTIFY=1 in your localdefs.mk if you wish to have this build option
enabled persistently.