In tls1_setup_key_block(), use the correct IV length for GCM mode, which
results in the key block length calculation also being correct. Rename a
number of variables so that their purpose becomes clear and simplify some
of the code.

Inspired by Adam Langley's chromium diffs.

ok miod@
Overhaul the key block handling in tls1_change_cipher_state() - use
meaningful variable names with pointer arithmitic, rather than n, i, j
and p with array indexing.

Based on Adam Langley's chromium diffs.

ok miod@