pass Pos to forceValue to improve infinite recursion error

(cherry picked from commit 8f418473)

Usually this just writes to stdout, but for ProgressBar, we need to
clear the current line, write the line to stdout, and then redraw the
progress bar.

(cherry picked from commit 696c0260)

More #3377.

We want to *trace* the 'Value *' arrays, not garbage-collect them!
Otherwise the vectors/maps can end up pointing to nowhere.

Fixes #3377. Closes #3384.

SourceExprCommand: allocate the vSourceExpr via uncollectable memory

NIX_USER_CONF_FILES

Motivation: maintain project-level configuration files.

Document the whole situation a bit better so that it corresponds to the
implementation, and add NIX_USER_CONF_FILES that allows overriding
which user files Nix will load during startup.

Previously the memory would occasionally be collected during eval since
the GC doesn't consider the member variable as alive / doesn't scan the
region of memory where the pointer lives.

By using the traceable_allocator<T> allocator provided by Boehm GC we
can ensure the memory isn't collected. It should be properly freed when
SourceExprCommand goes out of scope.

never use /var/folders for TMPDIR on darwin

This doesn't just cause problems for nix-store --serve but also results
in certain build failures. Builds that use unix domain sockets in their
tests often fail because the /var/folders prefix already consumes more
than half of the maximum length of socket paths.

    struct sockaddr_un {
       sa_family_t sun_family;               /* AF_UNIX */
       char        sun_path[108];            /* Pathname */
    };

Fix nix-build --check -K in sandbox w/o root

improve toFile error message when containing potential drv path

A test case for correct handling of temporary directory deletion that
was added to check.sh as part of PR #2689 was initially disabled for
Darwin because of a directory permission issue in PR #2688.

Now that the issue in PR #2688 is fixed, this commit enables the test
case for Darwin.

Temporarily add user-write permission to build directory so that it
can be moved out of the sandbox to the store with a .check suffix.

This is necessary because the build directory has already had its
permissions set read-only, but write permission is required
to update the directory's parent link to move it out of the sandbox.

Updated the related --check "derivation may not be deterministic"
messages to consistently use the real store paths.

Added test for non-root sandbox nix-build --check -K to demonstrate
issue and help prevent regressions.

Delete temporary directory on successful build

With --check and the --keep-failed (-K) flag, the temporary directory
was being retained regardless of whether the build was successful and
reproducible.  This removes the temporary directory, as expected, on
a reproducible check build.

Added tests to verify that temporary build directories are not
retained unnecessarily, particularly when using --check with
--keep-failed.

gc.cc: Ignore hidden files in temproots

Rename download to filetransfer