New MSVC property sheet for Multi-threaded XPDEV builds and usage:

defines XPDEV_THREAD_SAFE (not currently used in Windows builds) and
LINK_LIST_THREADSAFE (notice the inconsistent use of underscore :-)
Use of this property sheet fixes the problem with the terminal server global
variable 'uptime' getting corrupted when jsrt_GetNew() called listInit() from
js_rtpool.c which didn't #include sbbs.h and therefore had a different idea
about the size of link_list_t and thus corrupted the global data region
(zeroing out the 'uptime' variable, probably amonst other important things).