Removes all the TXG watching

Two reasons:
1) It has become impossible to do with reasonable accuracy on a default
installation of 0.6.3, and asking people to change a kernel module
variable to run the default tests seems a bit steep.
2) The numbers meaning is not as trivial to deduce as the speed in mib/sec
so just throwing it at people is not the best idea.
Adds get_txg_history_filename

During the ZoL 0.6.3 the file containing TXG data was moved. This
fucntion attempts to either tell you where it is, or trhow an error.
Change for linux_get_current_txg

Since they changed the location of the txg file, we are passing it in as
the arg instead of just the pool name. This allows the calling code to
cache the location of the file for repeated use.