libpq: Move cancellation related functions to fe-cancel.c

libpq: Move cancellation related functions to fe-cancel.c

In follow up commits we'll add more functions related to query
cancellations.  This groups those all together instead of mixing them
with the other functions in fe-connect.c.

The formerly static parse_int_param() function had to be exported to
other libpq users, so it's been renamed pqParseIntParam() and moved to a
more reasonable place within fe-connect.c (rather than randomly between
various keepalive-related routines).

Author: Jelte Fennema-Nio <jelte.fennema@microsoft.com>
Discussion: https://postgr.es/m/AM5PR83MB0178D3B31CA1B6EC4A8ECC42F7529@AM5PR83MB0178.EURPRD83.prod.outlook.com