_update to support version upgrades

[fsbruva]

We need to strictly declare the jail's version number, (or allow it to be specified) within the jail to be updated. Additionally, the target (what do we want to upgrade TO?) needs to be communicated.

There are two use cases for updates:

1. User has upgraded Nas4Free. Kernel will not match userland in jail, so we need to specify UNAME_r as the version that was actually installed. We need it to be a X.x-RELEASE. We also need to specify the destination (freebsd-update -r ?), probably by asking Nas4Free what version it is.
2. User is contemplating upgrading Nas4Free, and the jails match Nas4Free. Jail can be upgraded to the specified destination version. No manipulation of UNAME_r is required, as the current userland is the same as the kernel. However, we need to specify the destination version.

There is a third case - wherein the jail and Nas4Free do not match, and will remain that way after the "upgrade." This is if... Nas4Free is at 10.1, Jail is 9.1, and user wants jail to upgrade to 9.3. This use case will not be supported by TheBrig.