jail: define well-known parameter names in sys/jail.h

[seuros]

Summary

Userland jail managers hardcode jail parameter name strings; nothing
in the headers has ever named them, even when sys/jail.h already
has JAIL_META_PRIVATE/JAIL_META_SHARED for the metadata pair.

Added JAIL_PARAM_* string constants for the static parameters registered
by the base kernel.

No functional change.

MFC After: 1 week

[adrian]

i like this; it makes it much easier to write code that works by correctly using defines rather than hoping the strings match.

I'll approve it but let me first double check with phk or some other jails-y person before landing it.

[adrian]

(and subsequent commits should use these defines instead of hard-coded strings..)