Avoid building as root with `sudo make install`

When running `sudo make install`, we only want to run the actual install
as root, the building of the documentation and the distribution folder
should happen as the non-root user.

Related to #13728.
Add help diagnostic messages

This adds ‘help’ diagnostic messages to rustc. This is used for anything that
provides help to the user, particularly the `--explain` messages that were
previously integrated into the relevant error message.
Add rustup.sh to the repo

Just to have it somewhere to point to. Updating it will not
automatically update the one on static.rust-lang.org.
Fix several issues with the struct and enum representability-checking logic.
Tweak copy and formatting of Tasks guide
improve clone_from_slice performance
Link to Unicode SpecialCasing.txt document
Fix HTML version of the keyword table.

This should really fix rust-lang/rust#17528.
src/doc/README.md: fix a dead link.

Also make some typographical changes.
add intmax
Fix run-pass/lang-item-public test on FreeBSD
fix broken link in guide-strings.md
rustc: Warn about dead constants

A few catch-all blocks ended up not having this case for constants.

Closes #17925
Guide: remove promise of a counter in the guessing game.

Issue #17964.
Fix `find` check for executables

Apparently the fix in d08441b9 didn't catch a recent executable (#17965), but
I've verified that this does indeed catch the executable
Properly visit nested functions and closures in check_rvalues

This correctly catches moves of unsized values in nested
functions and closures.

Closes issue #17651
Fixes small error on the doc (task part)
Add regression test for issue #17651
Guide: specify that both shared and mutable borrows can be re-lent.
Doc: Fix C-Code Example in FFI Docs

Add missing void* for passed RustObject.