MFC r335575, r335786-r335787

r335575:
Use 'mv -f' in rc.d/ntpd to avoid spuriously halting the boot.

The final 'mv' to install a fetched leap-list file can fail (due to a
readonly fs, or schg flags, for example), and that leads to mv(1)
prompting the user, stopping the boot process. Instead, use mv -f
to supress the prompting, and if verbose mode is on, emit a warning
that the existing file cannot be replaced.

PR: 219255

r335786:
Rename variable ntp_tmp_leapfile to have a leading underbar, to distinguish
it from variables with similar names which are set in rc.conf. This will
make more sense as the script grows more similar-name local variables in
some upcoming changes.

r335787:
When being verbose about various leapfile versions, also mention expiration.

The expiration date is actually more of a version number than the version
date, because expiration changes twice a year, whereas the version only
changes when actual leap second events occur (except in USNO leapfiles,
which inappropriately bump the version with every expiration date change).