Implement Pkg.update hook

If your package contains a `deps/update.jl` file, it will now be executed
on Pkg.update().
Remove .git directory/repository of juliadoc in make install
Add unique(AbstractArray, dim)

Efficiently finds the unique columns, rows, etc. of an array. The
algorithm first hashes each row, then finds the unique hashes, and
finally checks that the hashes don't collide. It is roughly O(n) in
the number of elements in the matrix.

This is my first time using Cartesian. Without it, this code is
presently about 10% faster for finding unique rows of a matrix, but
the overhead is probably worth it for the generality.
probably fix #5818
@parallel documentation in stdlib
Merge pull request #5820 from amitmurthy/amitm/atpdoc

`@parallel` documentation in stdlib
unique(AbstractArray, dim): incorporate @timholy's suggestions
add an internal function to replace Main, use it in sysimg.jl

make the Core.Main binding non-constant, so uses of "Main" in general
refer to the *current* Main. Doing "import Main" will give you a
constant reference to the Main at that time.

fixes #5802

related to #1195
Issue #5455 : Added 1-norm, Inf-norm and Frobenius norm for SparseMatrixCSC
$(DESTDIR)$(prefix)/$(private_libdir) is redundant, should be $(DESTDIR)$(private_libdir)

I spent so much time introducing these Makefile variables, and yet I still have no idea how to use them
Update download links for win-extras

OpenSUSE's packaging moves quicker than ours!
Merge pull request #5717 from xhochy/no-.git-juliadoc

Remove .git directory/repository of juliadoc in make install
Revert "Update download links for win-extras"

This reverts commit af0675311655b8903ff5ecf7d12b184dec94766a.
Update download links for win-extras
Re-re-re update Rmath

Stop fighting me, git
Merge branch 'master' of https://github.com/JuliaLang/julia
Merge pull request #5811 from JuliaLang/sjk/uniquedim

Add unique(AbstractArray, dim)
Merge pull request #5822 from lruthotto/master

Issue #5455 : Added 1-norm, Inf-norm and Frobenius norm for SparseMatrixCSC
Merge pull request #5783 from JuliaLang/kf/updatehook

Implement Pkg.update hook
Ignore errors when removing doc .git files, as we might not have any (e.g. building from a tarball)