[Libguestfs] [PATCH libnbd] tests: Use libc_malloc_debug.so.0 instead of MALLOC_CHECK_ for glibc 2.34

This is the nbdkit change transplanted to libnbd. Because there are several subdirectory Makefiles which define the MALLOC_CHECK_ environment variables, I turned it into a macro which is placed in the top level subdir-rules.mk. Note this currently breaks valgrinding because of: https://sourceware.org/bugzilla/show_bug.cgi?id=28256 and because there's not a good place to unset GLIBC_TUNABLES in libnbd. Rich.