Avoid unintended macro expansion in KSORT_INIT_GENERIC

Netbsd's libc #defines uint16_t to __uint16_t (and similarly for
other stdint types).  This was expanded by KSORT_INIT_GENERIC()
resulting in functions being defined with slightly different names
compared to the ones produced by using KSORT_INIT() directly.
The names also no longer matched the results of expanding
ks_mergesort() and friends.

Fix this by adjusting where the underscore gets pasted into the
names.  This means KSORT_INIT_GENERIC can use the argument
in a token pasting operation, which prevents it from being
expanded.