preparations for user list in memory

preparations for user list in memory

The idea is to not lookup the user file/rsa key for
each connection/each buffer to be sent, but to load
it only once on startup.

This patch

	* renames list_add to para_list_add, as list_add is
	an exported symbol of mysql.h

	* makes get_key of crypt.c non-static and renames it to get_rsa_key.
	It is used from server.c to load the rsa key.

	* introduces struct _user, the new struct to hold user info. It is
	meant to replace the current struct user soon.

	* Introdoces init_user_list(), populate_user_list() and _get_user()
	in server.c. The latter is meant to replace get_user() of command.c

The new functions are not used yet.