m4: Check for sanlock_write_lockspace
    
    Currently, we are checking for sanlock_add_lockspace_timeout
    which is good for now. But in a subsequent patch we are going to
    use sanlock_write_lockspace (which sets an initial value for io
    timeout for sanlock). Now, there is no reason to check for both
    functions in sanlock library as the sanlock_write_lockspace was
    introduced in 2.7 release and the one we are currently checking
    for in the 2.5 release. Therefore it is safe to assume presence
    of sanlock_add_lockspace_timeout when sanlock_write_lockspace
    is detected.
    
    Moreover, the macro for conditional compilation is renamed to
    HAVE_SANLOCK_IO_TIMEOUT (as it now encapsulates two functions).
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>