Fix utc_offset_str for fractional TZs

Fix utc_offset_str for fractional TZs

Previously, for fractional offsets like TZ=FOO+3:3 (where the offset
is not an integer number of hours), utc_offset_str() would return a
large, incorrect integer value.  Adjust it to handle arbitrary
offsets, and document that it truncates any fractional seconds of the
offset.

Before this fix, whenever bup was operating with a fractional offset
(e.g. TZ=Australia/Adelaide), it wrote commits (i.e. saves) that it
won't be able to read.  Attempts to read them will fail with
"Exception: cannot parse commit".

Thanks to David Kettler for reporting the problem, and proposing an
alternate fix.

Reviewed-by: David Kettler <kettler@internode.on.net>
Tested-by: David Kettler <kettler@internode.on.net>
[rlb@defaultvalue.org: changed / to // for Python 3 compat]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>