time: fix to use format_time

[syusui-s]

Problem

It seems that format_time of tztime doesn't works correctly after v2.14.

If the configuration is written as follows, the string expected to be printed is "<span foreground='#ee9a00'>21-11-14 (Sun) 02:27:51</span>" but I got "%y-%m-%d (%a) %H:%M:%S".

tztime local {
  format = "<span foreground='#ee9a00'>%time</span>"
  format_time = "%y-%m-%d (%a) %H:%M:%S"
}

The detailed behavior of format_time is described in the doc: https://i3wm.org/docs/i3status.html#_tztime

Cause

The cause is that format is not correctly used.
format should be used when printing but format_time is used in v2.14.

The source code of the previous version v2.13 uses format.
https://github.com/i3/i3status/blame/101215bbc83b97de30b6b535bbe2e93d2e913804/src/print_time.c#L68-L77

This pull request fixes it.

Workaround

You can use % in format directly.

tztime local {
  format = "<span foreground='#ee9a00'>%y-%m-%d (%a) %H:%M:%S</span>"
}

Another issue

maybe_escape_markup was used in previous versions to sanitize date-time string formatted with format_time,

https://github.com/i3/i3status/blame/101215bbc83b97de30b6b535bbe2e93d2e913804/src/print_time.c#L74

Although, it doesn't care about the length of the buffer, I think it can lead to buffer overflow.

It seems that maybe_escape_markup is not used in the current implementation.
In other words, the current behavior is the same as if format_time was not used and strftime format was used in format.
I want to keep it as is and I think it is better to be fixed in the another pull request because I think it's rare that the output text is long, so I don't think it's a big problem as it is.

[byly]

This PR's commit fixes the bug for me (when both format_time and format are used in the tztime module).