Show session age in the "detached Mosh sessions" warning

[eminence]

Previously, the message just told you the pid of the detached mosh session:

Mosh: You have 6 detached Mosh sessions on this server, with PIDs:
        - mosh [3765640]
        - mosh [3940480]
        - mosh [3901319]
        - mosh [1216915]
        - mosh [3892360]
        - mosh [1623138]

But this doesn't immediately give you any hints about if this is an ancient session that's clearly from a dead process or not.

Now it looks like this:

Mosh: You have 6 detached Mosh sessions on this server, with PIDs:
        - mosh [3765640] (age 81d 20h 10m)
        - mosh [3940480] (age 63d 18h 5m)
        - mosh [3901319] (age 71d 3h 8m)
        - mosh [1216915] (age 13d 15h 9m)
        - mosh [3892360] (age 58d 11h 5m)
        - mosh [1623138] (age 10h 12m)

This change may reduce confusion that sometimes appears in issues like #394
Now it's more obvious if this session is part of a stale session or not