Remove residual blank line at start of Makefile

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

callout: retire callout_async_drain()

This function was used only in TCP before 446ccdd08e2a. It was born in
pain in 2016 to plug different complex panics in TCP timers. It wasn't
warmly accepted

callout: retire callout_async_drain()

This function was used only in TCP before 446ccdd08e2a. It was born in
pain in 2016 to plug different complex panics in TCP timers. It wasn't
warmly accepted in phabricator by all of the reviewers and my recollection
of overall agreement was that "if you need this KPI, then you'd better fix
your code to not need it". However, the function served its duty well all
the way to FreeBSD 14. But now that TCP doesn't need it anymore, let's
retire it to reduce complexity of callout code and also to avoid its
further use.

Reviewed by: jhb, markj, kib, rrs
Differential Revision: https://reviews.freebsd.org/D43546

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sys: Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Merge ^/head r317971 through r318379.

Mark this Makefile SUBDIR_PARALLEL

I inserted the necessary SUBDIR+= .WAIT in the previous commit

MFC after: 3 weeks
Sponsored by: Dell EMC Isilon

Add intermediary Makefile for compiling all items in the directory

MFC after: 3 weeks
Sponsored by: Dell EMC Isilon