Made 1st readBlock of Read/Write sessions "exact"
Both tapeFile::ReadSession and tapeFile::WriteSession start by reading VOL1 by calling Drive::readBlock() with a block of 84 bytes even though Logical Block Protection (LBP) is turned off at this early stage of a session. With LBP turned off, a drive will only return the exact record being read and wil not add 4 CRC bytes at the end. This means the drive will always return 80 bytes. Both tapeFile::ReadSession and tapeFile::WriteSessioas called Drive::readBlock() with a block of 84 bytes in order to work with an "in-house" version of mhvtl that tried to support LBP. Unfortunately the behaviour of this special version of mhvtl was incorrect because it would 84 bytes when LBP was disabled. The "in-house" version of mhvtl trying to support LBP is no longer used with CASTOR. This commit therefore simplfies the CASTOR code by replacing Drive::readBlock() with Drive::readExactBlock and by removing the conditional logic that dealt with the existance of an 84 byte VOL1 label in the presence of the "in-house" version of mhvtl.