Remove some bashisms from configure.ac

[Needs testing and review] Remove dead code

The manual reference was commented out, and the sole reference to this
program other than the configure script. Removed both.

"configure.ac" is used to generate "configure", which is supposed to be
run with /bin/sh (as suggested by the shebang line of "configure"). As a
result it is a good idea to remove any /bin/sh-incompatible syntax from
configure.ac. Otherwise, systems that do not use Bash as their /bin/sh
are unable to run "configure" due to syntax errors.

nix-store: remove debugging print

tests/fetchurl: fix after changing default hash from 512 to 256

Clarify nix-instantiate --read-write-mode

SRI hashes (https://www.w3.org/TR/SRI/) combine the hash algorithm and
a base-64 hash. This allows more concise and standard hash
specifications. For example, instead of

  import <nix/fetchurl.nl> {
    url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
    sha256 = "5d22dad058d5c800d65a115f919da22938c50dd6ba98c5e3a183172d149840a4";
  };

you can write

  import <nix/fetchurl.nl> {
    url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
    hash = "sha256-XSLa0FjVyADWWhFfkZ2iKTjFDda6mMXjoYMXLRSYQKQ=";
  };

In fixed-output derivations, the outputHashAlgo is no longer mandatory
if outputHash specifies the hash (either as an SRI or in the old
"<type>:<hash>" format).

'nix hash-{file,path}' now print hashes in SRI format by default. I
also reverted them to use SHA-256 by default because that's what we're
using most of the time in Nixpkgs.

Suggested by @zimbatm.

docs: add missing prerequisites: brotli, boost, libseccomp

nix repl: don't create result symlinks

probably typo

...at least MSVC unable to compile this

libstore: improve hash mismatch error messages

Use the same output ordering and format everywhere.

This is such a common issue that we trade the single-line error message for
more readability.

Old message:
```
fixed-output derivation produced path '/nix/store/d4nw9x2sy9q3r32f3g5l5h1k833c01vq-example.com' with sha256 hash '08y4734bm2zahw75b16bcmcg587vvyvh0n11gwiyir70divwp1rm' instead of the expected hash '1xzwnipjd54wl8g93vpw6hxnpmdabq0wqywriiwmh7x8k0lvpq5m'
```

New message:
```
hash mismatch in fixed-output derivation '/nix/store/d4nw9x2sy9q3r32f3g5l5h1k833c01vq-example.com':
  wanted: sha256:1xzwnipjd54wl8g93vpw6hxnpmdabq0wqywriiwmh7x8k0lvpq5m
  got:    sha256:08y4734bm2zahw75b16bcmcg587vvyvh0n11gwiyir70divwp1rm

```

https://hydra.nixos.org/build/85827920

nar-info-disk-cache: include ca in the cache entries

Without this information the content addressable state and hashes are
lost after the first request, this causes signatures to be required for
everything even tho the path could be verified without signing.

s3: make scheme configurable

This enables using for http for S3 request for debugging or
implementations that don't have https configured.  This is not a problem
for binary caches since they should not contain sensitive information.
Both package signatures and AWS auth already protect against tampering.

Solve hg "abandoned transaction" issue

download: if there are active requests, never sleep for 10s

repl: give user the choice between libeditline and libreadline

The goal is to support libeditline AND libreadline and let the user
decide at compile time which one to use.

Add a compile time option to use libreadline instead of
libeditline. If compiled against libreadline completion functionality
is lost because of a incompatibility between libeditlines and
libreadlines completion function. Completion with libreadline is
possible and can be added later.

To use libreadline instead of libeditline the environment
variables 'EDITLINE_LIBS' and 'EDITLINE_CFLAGS' have to been set
during the ./configure step.

Example:

  EDITLINE_LIBS="/usr/lib/x86_64-linux-gnu/libhistory.so /usr/lib/x86_64-linux-gnu/libreadline.so"
  EDITLINE_CFLAGS="-DREADLINE"

The reason for this change is that for example on Debian already three
different editline libraries exist but none of those is compatible the
flavor used by nix. My hope is that with this change it would be
easier to port nix to systems that have already libreadline available.

nix ls-nar: allow reading from FIFOs

fixes #2528