Add a new gptboot(8) man page.  Factor out the redundant information
in gpart(8) and boot(8), adding references to gptboot(8) in both.

Reviewed by:	jhb, ae, pjd, Paul Schenkeveld <bsdcan@psconsult.nl>, david_a_bright@dell.com (portions), gjb
MFC after:	1 week
Be sure to destory the fasttrap cleanup mutex when unloading the fasttrap
module. This should be MFCed with r250953.