Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Revert most part of 200420 as requested, as more review and polish is
needed.

Remove unneeded header includes from usr.bin/ except contributed code.

Tested with: make universe

Fix description of cmap_lookup_hard().

Add support for multibyte characters. The challenge here was to use
data structures that scale better with large character sets, instead of
arrays indexed by character value:
- Sets of characters to delete/squeeze are stored in a new "cset" structure,
which is implemented as a splay tree of extents. This structure has the
ability to store character classes (ala wctype(3)), but this is not
currently fully utilized.
- Mappings between characters are stored in a new "cmap" structure, which
is also a splay tree.
- The parser no longer builds arrays containing all the characters in a
particular class; instead, next() determines them on-the-fly using
nextwctype(3).