take a thread instead of a proc for their first argument.
- Add a mutex to protect the system-wide Linux osname, osrelease, and
oss_version variables.
- Change linux_get_prison() to take a thread instead of a proc for its
first argument and to use td_ucred rather than p_ucred. This is ok
because a thread's prison does not change even though it's ucred might.
- Also, change linux_get_prison() to return a struct prison * instead of
a struct linux_prison * since it returns with the struct prison locked
and this makes it easier to safely unlock the prison when we are done
messing with it.
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