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)

Previously the URI BNF didn't allow to specify empty
password. There should have been at least one symbol.

Now both user@host and user:@host forms are permitted.

In case of user@host, the password will parsed as nil.
In case of user:@host, the password will be parsed as
empty string.

Fixes #1512