Werk #7105: Check API: Make usage of dsname, unit, human_readable_func and infoname in check_levels clearer

Previously the dsname was also used as title if and only if the infoname was not stated. Therewith we got cryptic and not readable titles. The dsname should only be used for performance data variables.

Sometimes a title is not needed because the service description already gives the relevant information; especially in check plugins where there is just one {{thing}} to check, eg. in CPU services: Previously: CPU utilization OK - CPU utilization: 23.5% Now: CPU utilization OK - 23.5%

In general the structure is as following:

• If the infoname is specified the service detail is prefixed with this infoname as a title "TITLE: "
• If a human_readable_func is given the value and levels are converted with this function
• If a unit is given it is append to the value and each level.

Be aware: Every builtin human_readable_func already provides a unit. Thus the unit is not needed in general. There may be situations where a human_readable_func which provides a unit AND an additional unit info is useful, eg. when a bytes rate is calculated. In this case the service details should display "23.5 B/s" whereby C+ unit="/s", human_readable_func=get_bytes_human_readable, C-

If you use this API function {{check_levels}} in your check plugins you possibly need to adapt your check plugin functions. Otherwise duplicate unit infos may be displayed or there are missing titles ({{infoname}}).

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