setproctitle_fast()

Hello hackers,

FreeBSD's setproctitle() is a bit slow because it contains a syscall
or two, so people often run PostgreSQL with update_process_title set
to off on that OS. That makes the user experience not quite as nice
as Linux. As a weekend learn-me-some-kernel-hacking project I fixed
that and got the patch committed to FreeBSD 12, though I was asked to
use a new libc entry point _fast(). Here's a patch to teach
PostgreSQL about that. It doesn't have much effect on small systems,
but it makes "pgbench -c 40 -j 40 -S -M prepared" do ~10% more
transactions per second on an AWS m4.10xlarge instance.

I'll park this patch here until the FreeBSD feature escapes in a
RELEASE version in a few months.

For anyone interested, https://wiki.postgresql.org/wiki/FreeBSD has
some notes on this and other such things.

--
Thomas Munro
http://www.enterprisedb.com