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

SSL certificate search failed to find user profile certificates.

This ensures that they can't write to /nix/store. Fixes #2535.

manual: quote $servlets

manual: fix typo (s/gift/git/)

This reverts commit 5a1a8708. Counting
is hard.

Fix manual build

This was broken by some missing closing tags in 0bea4a50

Deprecate builtins.toPath

Closes #179.

preferLocalBuild does not in fact prevent substitution.

Since we're not using multi-part uploads at the moment, we can drop
this patch.

Fix typo in comments

config: use all of XDG_CONFIG_DIRS

Previously, config would only be read from XDG_CONFIG_HOME. This change
allows reading config from additional directories, which enables e.g.
per-project binary caches or chroot stores with the help of direnv.