Provide uni_realloc0() and uni_alloca()

[nicholatian]

uni_realloc0() does what uni_realloc() does, with the additional behaviour that if the new allocation is larger than the old, the newly-allocated memory will be zerofilled before it is returned to the caller. uni_alloca() does what alloca() does using compiler specifics, as this is not standard in ANSI C.