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

Remove $FreeBSD$: one-line sh pattern

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

gpart: append partition name to the underlying provider's physical path

If the underlying provider's physical path is null, then the gpart device's
physical path will be, too. Otherwise, it will app

gpart: append partition name to the underlying provider's physical path

If the underlying provider's physical path is null, then the gpart device's
physical path will be, too. Otherwise, it will append the partition name,
such as "/p1" or "/s1/a". This will make gpart work better with zfsd(8).

PR: 224965
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D14010

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