Implement granted privilege cache in struct user.

Reload user privileges from system spaces and
rebuild the cache whenever a grant or revoke occurs..

Implement revoking privileges from a role.
Basic tests pass.

Do not rebuild the privilege cache of objects
for which it can't have changed with a grant
or revoke.

Optimize rebuild_effective_grants().

Make role.test idempotent.

Make grant and revoke robust to malformed arguments.
Fix bugs in schema.lua, related to incorrect arguments
for grant/revoke.

Conflicts:
	doc/user/connectors.xml
	doc/user/stored-procedures.xml
	doc/user/tutorial.xml
	src/coio.cc
	src/evio.cc
	src/evio.h

Provide fine-grained keepalive options on Linux, which
supports per-socket defaults.

Fix a bug with trying to set keepalive options for UDP
sockets - don't try to do it.

Introduce 'public' role.
Auto-grant 'public' to every user.
Update the snapshot.

On ARM chars are unsigned

This code didn't worked on ARM, changing the table and parameters to int seems to work fine on any platform.

Track granted roles.
Represent roles granted to a user with a bitmap.
Disallow loops in role grants.
Add tests for box.schema.user.grant('username', 'rolename')
Add basic tests for loops in grant sequences.

Rewrite largely buggy code in authentication token lookup.
Do not waste space on authentication token bitmap, correctly
check for overflow of the bitmap.

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

A few renames.

Rewritten

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

Fixes gh-632.