Document multi-user installation, and add release notes about it not being the default on 2.1

manual: document why ~/.netrc doesn't work

add manual entries for disallowedRequisites and disallowedReferences

manual: distributed-builds: Mention - as default

release notes: note that the multi-user installer is available but not selected for Linux with systemd, and the bug about selinux

Use sh <(...) syntax for installation to preserve stdin and prompting

also update installation docs to account for changes in multi-user selection

docs: Add some examples to fetchGit

Print stats in more programs

This is already done by coerceToString(), provided that the argument
is a path (e.g. 'fetchGit ./bla'). It fixes the handling of URLs like
git@github.com:owner/repo.git. It breaks 'fetchGit "./bla"', but that
was never intended to work anyway and is inconsistent with other
builtin functions (e.g. 'readFile "./bla"' fails).

Fixes #2390.

https://hydra.nixos.org/build/80480356

docs: Add IDs to important sections

fix `error: unknown serve command 9`

Also some cosmetic improvements.

E.g.

  $ nix upgrade-nix
  error: directory '/home/eelco/Dev/nix/inst/bin' does not appear to be part of a Nix profile

instead of

  $ nix upgrade-nix
  error: '/home/eelco/Dev/nix/inst' is not a symlink

Docs: Fix install prefix

Fix a 32-bit overflow that resulted in negative numbers being printed;
use fmt() instead of boost::format(); change -H to -h for consistency
with 'ls' and 'du'; make the columns narrower (since they can't be
bigger than 1024.0).

If the user has an object greater than 1024 yottabytes, it'll just display it as
N yottabytes instead of overflowing.

Swaps to use boost::format strings instead of std::setw and std::setprecision.

Unfortunately, -h is already taken as a short option by --help, so we have to
use a different letter or the capitalized version.

Resolves #2363

This reduces the size of the closure by 45 MiB.

repl: don't add trailing spaces to history lines