When rerooting, take the init(8) path from argv[0] instead of fetching

When rerooting, take the init(8) path from argv[0] instead of fetching
it via kern.proc.pathname sysctl(2).  In some cases - booting from NFS
or rerooting after replacing the init binary with a new one - the sysctl
would fail.  In other cases - after upgrading, which moves the old init
to /sbin/init.bak - it would return /sbin/init.bak, which is the actual
path of the running init, instead of /sbin/init.

Reported by:	Melissa Jenkins <melissa-freebsd at littlebluecar.co.uk>, jilles@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation