Add check_user_share_access() which factors out the share security and read_only...

Add check_user_share_access() which factors out the share security and read_only flag setting code.

Allows this to be called from both make_connection_snum() as well as check_user_ok().
Gives a consistent share security check function.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>