rrdtool: Update to 1.8.0

rrdtool: Update to 1.8.0

For details see:
https://github.com/oetiker/rrdtool-1.x/releases/tag/v1.8.0

"Bugfixes

    python bindings: properly convert double values of rrd info
    failed to expand 'Py_UNUSED', Invalid usage when expanding 'Py_UNUSED'
    document --showtime in xport help output
    fix --use-nan-for-all-missing-data
    update rrdruby.pod
    add missing rrdruby.pod and rrdpython.pod to dist
    Set first_weekday to 0 (Sunday), when HAVE__NL_TIME_WEEK_1STDAY is not defined
    fix median calculation for all NaN inputs
    fix potential leak in xport during failure
    fix many warnings raised by Cppcheck
    fix many compiler warnings from latest gcc
    ensure proper initialization in rrd_daemon
    cleanup testsuite
    better testing
    avoid invalid read in rrd_client
    add symbols from rrdc to librrd
    Fix duplicate write_changes_to_disk() calls when HAVE_LIBRADOS is true and HAVE_MMAP is false
    documentation updates
    for SMIN example in docs
    fix for pyton3 compatibility
    freemem only for valid status <Christian Kr"oger>
    fix double meaning of time 0 as uninitialized value
    fix for zfs not supporting fallocate. this makes resize work on zfs
    add rrdrados.pod to dist
    fetch - do not call rrd_freemem on uninitialized pointers
    use separate pango fontmap per thread
    switch to python 3
    do not leak filename when opening a broken file
    fix leaks in rrdcached
    avoid segfault when flushing cache
    escape json in legend entries
    fix leak in xport
    make rrdcgi param parsing more robust
    fix race in journal_write"

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>