Fix "Strange output for `box.info.status`: loading"

txn_rollback started to yield recently. Since it was invoked via a ffi
call, which must never yield, all sorts of bad things started to happen.

https://github.com/tarantool/build/issues/16

* the bug with iobuf recycling has always been in 1.6,
  and is fixed in 1.7, but got more exposure with the patch
  for gh-846. "Idle" state of iobuf was calculated incorrectly.

Package.searchpath, if probed XXX.dylib and failed will attempt
XXX.so before moving to next path element (osx).

Fixes #810

* don't recycle input buffer if this leads to growth of the output
buffer

tarantool/libyaml was re-created directly from hg repo
using hg-fast-export. Code is not changed.

tx_process_join_subscribe() attempted to write response to closed socket.
This patch also fixes diag_raise() with SocketError.

Fixes gh-1624.

Fixes #1617

Fixes debian/#831361

Free the vclock_to_string() buffer in all cases.

Incorporate recff_cdata_arith fix.

 * Pcall bugfix
 * fiber context switch

This happens because console:eval swallows errors
and doesn't print them to stdout.

Also, console.connect() doesn't always call on_client_disconnect()
in case of errors, so the code that was meant to terminate local
session on errors, didn't do this in some cases and caused local
console to remain open.

Fixes #1586

Fixed Lua code block formatting in box_index.rst

Review fixes:
* revert changes in console.lua, they should be pushed down to
  net.box (it should accept empty passwords)