fe-utils - share query cancellation code

Hello Devs,

This patch moves duplicated query cancellation code code from psql & 
scripts to fe-utils, so that it is shared and may be used by other 
commands.

This is because Masao-san suggested to add a query cancellation feature to 
pgbench for long queries (server-side data generation being discussed, but 
possibly pk and fk could use that as well).

-- 
Fabien.