Move OTP sockets to KDC_RUN_DIR

Some system configurations expect Unix-domain sockets to live under
/run or /var/run, and not other parts of /var where persistent
application state lives.  Define a new directory KDC_RUN_DIR using
$runstatedir (new in autoconf 2.70, so fall back to $localstatedir/run
if it's not set) and use that for the default socket path.

[ghudson@mit.edu: commit message, otp.rst formatting fix]

ticket: 7859 (new)