res_resolver_unbound: Allow compilation with libunbound version < 1.5

libunbound at version 1.4.20 (which CentOS still uses) declared all
of their string function parameters as as 'char *'.  1.4.21 changed
them all to 'const char *'.  Thankfully 1.4.21 also introduced the
UNBOUND_VERSION_MAJOR define so configure now checks for that and
sets HAVE_UNBOUND_CONST_PARAMS.  res_resolver_unbound then checks
that and casts away the 'const' if it's not set.

Tested compile and testsuite on CentOS6 (1.4.20), Ubuntu14 (1.4.22) and
Fedora24 (1.5.4).  There are a few failing tests to be addressed though.

ASTERISK-26283 #close

Change-Id: Ib708b19b706c5d0ba7b7d5473e6df339d9ae4148