Fix remaining documentation to reflect fail!() -> panic!()

Throughout the docs, "failure" was replaced with "panics" if it means a
task panic. Otherwise, it remained as is, or changed to "errors" to
clearly differentiate it from a task panic.
Fix documentation bug

The first paragraph must be separated from the next paragraph.
Otherwise, rustdoc will consider the content of the latter as part of
the title.
Results aren't panics

A typo about Results being panics crawled in. Fixing it.
Fix po4a.conf
fix "warning: deprecated syntax, use `for` keyword now"
Reference: Fix list in Expressions section
auto merge of #18880 : barosl/rust/doc-fail-to-panic, r=alexcrichton

I found some occurrences of "failure" and "fails" in the documentation. I changed them to "panics" if it means a task panic. Otherwise I left it as is, or changed it to "errors" to clearly distinguish them.

Also, I made a minor fix that is breaking the layout of a module page. "Example" is shown in an irrelevant place from the following page: http://doc.rust-lang.org/std/os/index.html
auto merge of #18894 : ArtemGr/rust/patch-1, r=pnkfelix

A typo about Results being panics crawled in. Fixing it.
auto merge of #18901 : steveklabnik/rust/quickfix, r=alexcrichton

Small copy/paste error from the crates guide.
auto merge of #18922 : japaric/rust/for, r=jakub-

r? @alexcrichton
auto merge of #18924 : cakebaker/rust/fix_list, r=steveklabnik
Remove Runtime trait and bookkeeping module

This commit removes most of the remaining runtime infrastructure related
to the green/native split. In particular, it removes the `Runtime` trait
and instead inlines the native implementation.

Closes #17325

[breaking-change]
Remove libnative

With runtime removal complete, there's nothing left of libnative. This
commit removes it.

Fixes #18687

[breaking-change]
Remove libgreen

With runtime removal complete, there is no longer any reason to provide
libgreen.

[breaking-change]
Rewrite sync::mutex as thin layer over native mutexes

Previously, sync::mutex had to split between green and native runtime
systems and thus could not simply use the native mutex facility.

This commit rewrites sync::mutex to link directly to native mutexes; in
the future, the two will probably be coalesced into a single
module (once librustrt is pulled into libstd wholesale).
Fallout from libgreen and libnative removal
Make most of std::rt private

Previously, the entire runtime API surface was publicly exposed, but
that is neither necessary nor desirable. This commit hides most of the
module, using librustrt directly as needed. The arrangement will need to
be revisited when rustrt is pulled into std.

[breaking-change]
Fallout from new termination semantics