rustc: Forbid plugin_registrar in only rlib form

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475
Some documentation fixes and improvements
install: Make the LD_LIRARY_PATH warning more aesthetic
install: Run ldconfig when installing on Unix. Closes #15596.

If ldconfig fails it emits a warning. This is very possible when installing
to a non-system directory, so the warning tries to indicate that it may
not be a problem.
rustdoc: Add stability dashboard

This commit adds a crate-level dashboard summarizing the stability
levels of all items for all submodules of the crate.

The information is also written as a json file, intended for consumption
by pages like http://huonw.github.io/isrustfastyet/

Closes #13541
Bug fixes for stability tracking

This commit adds correct stability tracking for struct fields and
corrects some places where rustdoc was not pulling the stability data.
auto merge of #15565 : alexcrichton/rust/issue-15475, r=huonw

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475
auto merge of #15586 : aturon/rust/stability-dashboard, r=alexcrichton

This PR adds a crate-level dashboard summarizing the stability levels of all items for all submodules of the crate.

The information is also written as a json file, intended for consumption by pages like http://huonw.github.io/isrustfastyet/

Along the way, fixes a few bugs in stability tracking and places where rustdoc was not pulling the existing stability data.

Closes #13541
auto merge of #15589 : Denommus/rust/master, r=alexcrichton

I have noticed some errors and some absences that I considered essential to the usability of rustc in zsh, so I included them and updated some of the man page.
Only run ldconfig on Linux
auto merge of #15597 : brson/rust/ldconfig, r=pcwalton

If ldconfig fails it emits a warning. This is very possible when installing
to a non-system directory, so the warning tries to indicate that it may
not be a problem.
Auto-merging 8a2b7a5

Travis: Passed at 2014-07-12T08:03:26Z
https://travis-ci.org/vhbit/rust/builds/29758374