Simplified quick_abort_pct code and improved its docs (#1921)

Simplified quick_abort_pct code and improved its docs (#1921)

Instead of ignoring quick_abort_pct settings that would, together with
other conditions, abort a pending download of a 99-byte or smaller
response, Squid now honors quick_abort_pct for all response sizes. Most
Squids are not going to be affected by this change because default
quick_abort_min settings (16KB) prevent aborts of 99-byte responses even
before quick_abort_pct is checked.

Due to conversion from integer to floating point math, this change may
affect responses larger than 99 bytes as well, but these effects ought
to be limited to cases where the decision is based on a tiny difference
(e.g., receiving 1% more bytes would have triggered full download). In
most such cases, the decision could probably go either way due to
response header size fluctuations anyway.

Also updated quick_abort_pct documentation, primarily to clarify a
misleading statement: Squid did not and does not treat 16KB or smaller
responses specially in this context. The original statement was probably
based on quick_abort_min _default_ setting of 16KB, but statement
phrasing and placement hid that connection.