Enable pthreads for new libraries

nix develop: Support chroot stores

Fix the generation of nix3 manpages

Fixes #5024.

- Separate the generation of the manpages from their installation
- Make sure that `make` generates the manpages
- Make sure that `make install` installs them

Fix #5051

--eval-store and faster closure copying

Fix --no-gc-warning

libutil: use uniform initialization in _deletePath

libexpr: Remove unused code

Signed-off-by: Pamplemousse <xav.maso@gmail.com>

Avoid global counters

This adds a new store operation 'addMultipleToStore' that reads a
number of NARs and ValidPathInfos from a Source, allowing any number
of store paths to be copied in a single call. This is much faster on
high-latency links when copying a lot of small files, like .drv
closures.

For example, on a connection with an 50 ms delay:

Before:

  $ nix copy --to 'unix:///tmp/proxy-socket?root=/tmp/dest-chroot' \
    /nix/store/90jjw94xiyg5drj70whm9yll6xjj0ca9-hello-2.10.drv \
    --derivation --no-check-sigs
  real    0m57.868s
  user    0m0.103s
  sys     0m0.056s

After:

  real    0m0.690s
  user    0m0.017s
  sys     0m0.011s

Broken by 8e758d40

Otherwise I get a compiler error when building for NetBSD:

src/libutil/util.cc: In function 'void nix::_deletePath(const Path&, uint64_t&)':
src/libutil/util.cc:438:17: error: base operand of '->' is not a pointer
  438 |     AutoCloseFD dirfd(open(dir.c_str(), O_RDONLY));
      |                 ^~~~~
src/libutil/util.cc:439:10: error: 'dirfd' was not declared in this scope
  439 |     if (!dirfd) {
      |          ^~~~~
src/libutil/util.cc:444:17: error: 'dirfd' was not declared in this scope
  444 |     _deletePath(dirfd.get(), path, bytesFreed);
      |                 ^~~~~

Otherwise the lack of pthread causes linking to fail for NetBSD.

installer: fix --no-modify-profile help text

installer: update global nix.conf location

With this, we don't have to copy the entire .drv closure to the
destination store ahead of time (or at all). Instead, buildPaths()
reads .drv files from the eval store and copies inputSrcs to the
destination store if it needs to build a derivation.

Issue #5025.

In particular, this now works:

  $ nix path-info --eval-store auto --store https://cache.nixos.org nixpkgs#hello

Previously this would fail as it would try to upload the hello .drv to
cache.nixos.org. Now the .drv is instantiated in the local store, and
then we check for the existence of the outputs in cache.nixos.org.

If a store doesn't support GC, then we don't need to warn about the
inability to create roots.

Signed-off-by: Pamplemousse <xav.maso@gmail.com>

Also remove an erroneous comment.