Add casts to work around harmless -Werror warnings from clang 10.0.0,

Add casts to work around harmless -Werror warnings from clang 10.0.0,
such as:

usr.sbin/timed/timed/networkdelta.c:160:13: error: implicit conversion from 'long' to 'float' changes value from 9223372036854775807 to 9223372036854775808
      [-Werror,-Wimplicit-int-float-conversion]
        float ap = LONG_MAX;            /* bounds on the median */
              ~~   ^~~~~~~~

Direct commit to stable/{10,11,12}, since timed has been removed from
FreeBSD 13.

git-svn-id: svn://svn.freebsd.org/base/stable/10@360338 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f