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Small improvements and fixes

* fix qtype usage
* send an independent first request with multistreams
* fix average time with multistreams
* add comments

See merge request bortzmeyer/homer!19

When using multistreams, requests are sent in parallel. curl keeps
track of different time values for each handle independently. This
means that we can retrieve the transfer duration for each handle
separatly.
It is then possible to calculate the average time taken by each
transfer independently. Even so this might be an interesting data to
compute, this does not reflect the benefits of using parallelism.
Therefore the average time taken by a request is calculate as the
total time elapsed to perform all the requests divided by the number
of transfers.

When using curl multi, we populate the multi with handles and then
call multi_perform which leads to curl establishing the multi
connection and sending tranfers once it is done. This looks
satfisfying enough if we don't look at the time values stored by curl
in each handles. In this case it appears that all the transfer time
are set to the time the curl multi was started meaning that all
transfers keep track of the connection time, this is normal for the
first transfer but unnecessary for all the following ones.

Therefore by sending a first transfer asking for the root NS, it
allows to open the connection without populating the resolver cache.
And thus keeps the curl timers more accurate.

[DoT] Pipelining

See merge request bortzmeyer/homer!18

The OK/KO output was reserved for the --check option. When using
--pipelining it could be nice to say if it succeed or fail (at
this point success only means that all the queries got an answer
in the given time window)

Explain options in usage

See merge request bortzmeyer/homer!17

Options' output refactored

See merge request bortzmeyer/homer!16

When using monitoring, the exit call is part of the print_result
function, making these last tests before exiting unnecessary

HTTP/2 streams

Closes #26 and #8

See merge request bortzmeyer/homer!11

Remove short option for debug

See merge request bortzmeyer/homer!15

Short option -d is already in use as the short option
for --delay

Option debug

Closes #25

See merge request bortzmeyer/homer!14