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Fixes issue in #2313 found by @FrenkelS which broke Elksdoom.
fmemcpynow saves DS (and ES for good measure). The same fix is applied tofmemcmp.Within Watcom ASM functions, it seems DS needs to be saved if altered, not sure whether ES needs saving or not. I haven't been able to generate any C code that depends on DS (or ES) being saved across a function call. Nonetheless, without saving DS in fmemcpy, Elksdoom crashes.
There seems to be some confusion in whether the segment registers should or must be saved across function calls; not sure whether this is a singular issue with Doom and ASM code, something required for all ASM functions mixed with C, or only required when mixing ASM functions with unknown ASM functions. The Open Watcom C User's Guide states that saving and restoring segment registers is not normally emitted by the C compiler unless the -Wc,-r option is set, and caution should be exercised, although the warning seems related to protected mode selectors only: