#638, checking/configuring in cmake pthread_yield and sched_yield; READLINE_ROOT for better readline control;

Rewritten

Write a function which can be used to check whether a user
exists or not.

Fixes gh-632.

Users should be able to specify some flags in CC or CXX variables
instead of putting it to to C/CXXFLAGS (e.g. CC="cc --sysroot=xx -m32").
We faced with this problem on Intel Edison/Galileo SDK.

added comments to rtree, added more misc tests for rtree, moved key validation from index to key_def, added NUMBER key type

If no password is set for GUEST user, the authentication
request must not contain auth data, only user name.

If no password is set for GUEST user, allow a client
to authenticate to GUEST.

For all other users, no password means that it's only possible to become
such user with session.su() or inside a setuid function.

The exception for GUEST user is useful for connection
pooling, where we would like to "reset" a connection to its
default, non-authenticated state, when returning it to the pool.

Weed out 'box' data type for now. A special name can backfire
in the future, when we exteend HASH/TREE/other types.

Add checks for uniqueness.

Disable check of key parts, it was not used any way,
since an explicit check is made in initIterator() and findByKey().

Mark sophia engine as  not supporting RTREE.

Fix a bug in iovec_join which would overwrite data of
the first iovec with subsequent iovecs.
Double the default buffer size.