uma: reorganize flags

• Garbage collect UMA_ZONE_PAGEABLE & UMA_ZONE_STATIC.
• Move flag VTOSLAB from public to private.
• Introduce public NOTPAGE flag and make HASH private.
• Introduce public NOTOUCH flag and make OFFPAGE private.
• Update man page.

The net effect of this should be to make the contract with clients more
clear. Clients should choose constraints, UMA will figure out how to
implement them. This also breaks the confusing double meaning of
OFFPAGE.

Reviewed by: jeff, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23016