Fix overflow in pg_size_pretty

Hi all,

Attached is a patch that resolves an overflow in pg_size_pretty() that
resulted in unexpected behavior when PG_INT64_MIN was passed in as an
argument.

The pg_abs_s64() helper function is extracted and simplified from patch
0001 from [0]. I didn't add similar functions for other sized integers
since they'd be unused, but I'd be happy to add them if others
disagree.

`SELECT -9223372036854775808::bigint` results in an out of range error,
even though `-9223372036854775808` can fit in a `bigint` and
`SELECT pg_typeof(-9223372036854775808)` returns `bigint`. That's why
the `::bigint` cast is omitted from my test.

[0]
https://www.postgresql.org/message-id/flat/CAAvxfHdBPOyEGS7s+xf4iaW0-cgiq25jpYdWBqQqvLtLe_t6tw(at)mail(dot)gmail(dot)com

Thanks,
Joseph Koshakow