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.

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.

NIX_USER_CONF_FILES

SourceExprCommand: allocate the vSourceExpr via uncollectable memory

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.

More #3377.

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 696c026)

(cherry picked from commit 8f41847)