[cmyth] Handle UTC timestamp

Changes allow to handle UTC times. Since Myth protocol 0.26 commands can return time values using
format ISO8601 and containing the zone flag 'Z'. Until now no help to handle them. Here i add tools
to process them.

I banned the conversion from UTC timestamp to 'time_t' type. Once timestamp created as UTC time, it
can be formatted as string or other for processing only.

Add new attribut 'timestamp_isutc' equal 1 when timestamp contains UTC time. UTC timestamp can be
create by two ways:
- First using function cmyth_timestamp_utc_from_unixtime().
- Second using function cmyth_timestamp_from_string(). Here if the string is formatted using standard
ISO8601 and constains 'Z' after seconds the function flags 'timestamp_isutc'.

Update 'to_string()' and 'to_display()' functions to consider timestamp_isutc.
Prohibit conversion in all functions using 'mktime()' by returning NULL or (time_t) -1.
Add new function 'cmyth_timestamp_to_utc()' to convert local timestamp to UTC timestamp.
Add new function 'cmyth_timestamp_isutc()' to retreive the zone type of timestamp: 1 = UTC , 0 = local