• fix service script

For some reason the PID written to the pidfile by htpdate does not match its
actual PID, which renders the standard PID-handling mechanism of rc.subr(8)
useless.

As a solution, depend only on the process name to figure out the PID of the
running process.

NB: the old pidfile has to be removed before starting htpdate. Otherwise,
the program complains about the pidfile already existing (which htpdate
interprets as another instance of htpdate already running).

Also:

• Use %%PREFIX%% to let the ports provide a proper path for the binary.
• Do not specify htpdate_flags in command_args. htpdate_flags are already passed to the program by rc.subr(8).

In response to this mailing list thread:
https://lists.freebsd.org/pipermail/freebsd-rc/2019-August/004047.html

Submitted by: 0mp@
Differential Revision: https://reviews.freebsd.org/D21431