pg_upgrade doc: link mode additions

Mention that link mode uses less disk space, and uses junction points on
Windows.

Backpatch to 9.3.
pgbench: Remove stray use of "float" math.

Oversight in commit 4a87f308b33457670f9ab4bc622678e5d285b9c2.

Fabien COELHO
pgbench: Elaborate latency reporting.

Isolate transaction latency (elapsed time between submitting first
command and receiving response to last command) from client-side delays
pertaining to the --rate schedule.  Under --rate, report schedule lag as
defined in the documentation.  Report latency standard deviation
whenever we collect the measurements to do so.  All of these changes
affect --progress messages and the final report.

Fabien COELHO, reviewed by Pavel Stehule.
Fix various bugs in postmaster SIGKILL processing

Clamp the minimum sleep time during immediate shutdown or crash to a
minimum of zero, not a maximum of one second.  The previous code could
result in a negative sleep time, leading to failure in select() calls.

Also, on crash recovery, reset AbortStartTime as soon as SIGKILL is sent
or abort processing has commenced instead of waiting until the startup
process completes.  Reset AbortStartTime as soon as SIGKILL is sent,
too, to avoid doing that repeatedly.

Per trouble report from Jeff Janes on
CAMkU=1xd3=wFqZwwuXPWe4BQs3h1seYo8LV9JtSjW5RodoPxMg@mail.gmail.com

Author: MauMau
pgbench: Comment on thread timing hazards.

Reviewed by Fabien COELHO.
Document support for VPATH builds of extensions.

Cédric Villemain and me.