[watcom] Save DS register across syscalls, don't set DS on NULL pointers #2171
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The DS register would be set to zero when NULL was passed to system calls in the OpenWatcom version of the ELKS C library. This then allowed a NULL DS register to be write outside the process address space in the previous commit.
This PR now saves DS across Watcom system call wrappers using the OWC -Wc,-r option so this won't ever happen. In addition, the sys_setseg macro checks for NULL and won't set DS if so.
All pointer arguments to Watcom syscalls are now checked/set with sys_setseg, and the OWC syscall wrappers cleaned up.
A Watcom ASM version of fmemcmp is added to the C library, although still unused by 8086 toolchain as an inline version was written within AS86 for highest speed.