[master] Improved parsing error checking for subnet address/prefix values

    Merge branch 'trac5418'

diff --cc src/bin/dhcp4/tests/config_parser_unittest.cc
Simple merge

diff --cc src/bin/dhcp6/tests/config_parser_unittest.cc
Simple merge

diff --cc src/lib/dhcpsrv/parsers/dhcp_parsers.cc
Simple merge