src/etc/local_stage0.sh: remove reference to libextra.

Signed-off-by: NODA, Kai <nodakai@gmail.com>
mk/dist.mk: install LICENSE.txt on Linux.

install.sh copies it to <prefix>/share/doc/$(PKG_NAME)/ where
$(PKG_NAME) is something like "rust-0.13.0".

It is generated by concatenating LICENSE, LICENSE-MIT, LICENSE-APACHE.
and AUTHORS.txt.  It used to include only the first three, but including
AUTHORS.txt is preferable because LICENSE mentions it.

Signed-off-by: NODA, Kai <nodakai@gmail.com>
COPYRIGHT: clarify rustc includes OpenBSD regex code.

Signed-off-by: NODA, Kai <nodakai@gmail.com>
librustc_trans and librustdoc: add license::print().

The license document is compressed by DEFLATE and then stored as
pub const license::text::COPYRIGHT of type &'static [u8].
A helper script src/etc/license2rs.sh takes care of updating it.

It is decompressed by flate::inflate_bytes() on runtime.

Fix #17690

Signed-off-by: NODA, Kai <nodakai@gmail.com>