Improve portability

- Use clearenv() if it exists
- Don't rely on <limits.h> being pulled in by <sys/param.h>
- Rename the DEFAULT_TYPE macro in sesrun.c.  This name appears to be
  used on Solaris. It's not a good choice.

configure.ac

@@ -211,7 +211,7 @@ AM_COND_IF([DEVEL_DEBUG],
 AC_SEARCH_LIBS([setusercontext], [util])
 
 # Define HAVE_XXXXX macros for some system functions
-AC_CHECK_FUNCS([setusercontext getgrouplist])
+AC_CHECK_FUNCS([setusercontext getgrouplist clearenv])
 
 # The type used by getgrouplist() is the same type used by getgroups()
 AC_TYPE_GETGROUPS