MFH: r260483, r260484, r260594, r260595, r260596, r260597

MFH: r260483, r260484, r260594, r260595, r260596, r260597

Improve error message shown to the user when trying to load a module that is
already loaded or compiled withing the kernel
Point the user to dmesg(1) to get informations about why loading a module did fail
instead of printing the cryptic "Exec format error"
Update the BUGS section of kld(4) according the recent changes in kldload(8)

git-svn-id: svn://svn.freebsd.org/base/stable/10@260906 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f