Allow use of libbsd functions with configure option --with-libbsd

Allow use of libbsd functions with configure option --with-libbsd

Add a new configure option `--with-libbsd', which allows to use libbsd's
portable implementations of:

    strlcpy strlcat arc4random arc4random_uniform reallocarray

instead of the embedded code copies in contrib/, which will be
difficult to maintain in the long term.

Also patch util/random.c so that, when building with libbsd and without
OpenSSL, arc4random can still be used as the PRNG.  Otherwise, building
with libnettle would need a kernel-specific getentropy implementation,
and libbsd does not export one.

[edmonds@debian.org: Imported patch description from BTS, refreshed
patch against Unbound 1.9.6.]