Print pgbench results on SIGINT

Hi hackers!

While working on [1] or [2] I sometimes use pgbench in short-run mode:
see if there is some performance improvements or degradation for 5-10 seconds,
press CTRL+C, make some code changes and repeat the measurements.

For now, pgbench prints nothing on CTRL+C, which hides the run statistics
from user. What if it printed the collected statistics anyway, along with the
existing 'Run was aborted; the above results are incomplete.' message?

Attached POC-patch does:
- on first SIGINT stops accepting new transactions, discards the in-flight ones
and print the summary as usual, exiting with status 130
- on second SIGINT fallbacks to standard handler immediately
- caps all wait at one second, so the SIGINT can interrupt the \sleep

For now I am not sure about exit code..
Any feedback would be appreciated :)

[1] - https://github.com/yandex/odyssey
[2] - https://github.com/pg-sharding/spqr/

--
Best regards,
Roman Khapov