Here is another pack of changes about gathering common client code in
sql-common/client.c.
Now i symlink the client.c from sql/ and libmysql/. These directories
have client_settings.h files to be included to client.c. It contains
defines and declarations to compile client.c in appropriate manner.
Also i've added include/sql_common.h, containing declarations of what
is exported from client.c

I removed as many #ifdef-s from client.c as i dared to. I think it's better
push it with some extra #ifdef-s now (of course, if everythihg besides it is
ok) so other people can check the code.