cgi/graph: replace Date::Manip with Date::Parse

As stated in #1389 by terryburton :

    munin-cgi-graph invokes UnixDate from Date::Manip which invokes the
    system's date command under a shell. This makes it difficult to create a tight
    MAC security profile (such as AppArmor or SELinux) since it becomes necessary
    to permit Munin to exec a shell, which is a gaping hole.

And cbiedl added :

    Also due to the shell execution, Date::Manip is sloooow. On my small
    test box, the entire Date::Manip::ParseDate and ::UnixDate line
    takes 120 ms, after replacing it with Date::Parse::str2time it is
    down to 3ms(sic!). Keep in mind this code is run for every graph a
    browser requests.

    Another issue mit Date::Manip is its API change, and at some point in
    the future the legacy DM5 munin uses might be obsoleted anyway.

    It should be noted Date::Parse::str2time understands all three
    timestamp formats as required by RFC 2616 in "3.3.1 Full Date". So if
    there's really breakage due to weird date strings specified by a
    client, it's rather their fault.

Kudos to terryburton for the initial patch. I only edited it a little and then
rebased it onto 2.0.6, so it can be also merged directly on the stable-2.0
branch. I also added the full commit message. This is to avoid having to look
at the ticket, as the rationale behind the change is solid, but might be subtle.

Closes: #1389
Merge branch 'fix-1389' into devel

Conflicts:
	master/_bin/munin-cgi-graph.in