gmirror.8: Remove references rc.early

The manual page of gmirror describes how gmirror providers can be used
for kernel dumps. Unfortunately, the instruction references
/etc/rc.early, which is no longer a part of rc(8).

Remove references to rc.early and suggest creating an rc(8) service
script instead.

Future work: In the Problem Report on Bugzilla, Lawrence Chen suggested
adding example rc(8) scripts to the gmirror. However, those examples
need to be tested before they become official reference examples in the
base. Also, those scripts should probably land directly to /etc/rc.d,
/usr/share/examples/rc.d, or /usr/share/examples/gmirror instead of the
gmirror manual page.

PR: 178818
Reported by: Lawrence Chen <beastie@tardisi.com>
Fixes: dd2b024a336f Removal of early.sh
MFC after: 1 week

(cherry picked from commit a848315f68047ae46a5d96cbc7a890e645ce646f)