[#3210] refactor string utilities

- Rename util/strutil.h to util/str.h to escape redundancy.
- Simplify trim function.
- Remove unused functions.
- Get rid of the regex conditional compilation that helped Kea build
  with ancient compilers. Lack of proper regex functionality now results
  in failure in configure.ac.

src/bin/dhcp4/json_config_parser.cc

@@ -42,7 +42,7 @@
 #include <process/config_ctl_parser.h>
 #include <util/encode/encode.h>
 #include <util/multi_threading_mgr.h>
-#include <util/strutil.h>
+#include <util/str.h>
 
 #include <boost/algorithm/string.hpp>
 #include <boost/lexical_cast.hpp>