"nm-utils/nm-shared-utils.h" shall contain utility function without other
dependencies. It is intended to be used by other projects as-is.

nm_utils_random_bytes() requires getrandom() and a HAVE_GETRANDOM configure
check. That makes it more cumbersome to re-use "nm-shared-utils.h", in
cases where you don't care about nm_utils_random_bytes().

Split nm_utils_random_bytes() out to a separate file.

Same for hash utils, which depend on nm_utils_random_bytes(). Also, hash
utils will eventually be extended to use siphash24.