r40: config.h.in, configure.in: get rid of malloc() and realloc()

r40: config.h.in, configure.in: get rid of malloc() and realloc()
	tests. They test whether malloc behaves as expected when doign
	something we don't do anyway. sf.net bug#824073
nbd-server.c:
	* realloc() the right way. sf.net bug#824092
	* incorporated parts of patch#815093 (some parts broke things
	  which weren't always compiled-in, so didn't do those)