Merge and remove duplicate and divergent files

There are several files that were either copied and then diverged, or that are intended to be compiled twice with different flags.

Specifically:
mysys/my_lock.c|sql/my_lock.c
sql/sql_string.cc client/sql_string.cc
libmysql/client.c
libmysql/net_serv.c

my_lock and sql_string just need to be consolidated and moved to mysys and mystrings respectively.
net_serv.c and client.c need to be fixed so that we can just compile them into libmysqlclient and then link mysqld with libmysqlclient directly.