jail: Simplify list of methods and move explanations to poudriere.8
jail: Give a hint when there is a missing param
jail: Add a tar=PATH method

Ticket:	[d6f71fe708]
Fix unused variable warnings

Submitted by:	sbruno
jail -c: Always auto update with freebsd-update when using ftp|http|ftp-archive
Generalize qemu support to use binmiscctl

Submitted by:	sbruno
jail -cu: Only require emultation if needed and also set EMULTATION for updating
jail: Fix support for uname -m != uname -p and support both -a TARGET_ARCH and -a TARGET.TARGET_ARCH

Ticket:		[def3b06fe1]
Reported by:	jhibbits
jail: Sort method list
Rename private dir /npkg -> /.npkg
jail -c: Do more of the jail setup on-the-fly at build startup
jail: Add -m null to import -M <path> as an existing jail to use