Count null_bits separately from field offsets and adjust them in case of primary key parts.
(Previously a CREATE TABLE with a lot of null fields that was part of a primary key caused MySQL to wrongly count the number of bytes needed to store null bits)
This is a more complete bug fix for #6236


mysql-test/r/alter_table.result:
  More test for bug #6236 (CREATE TABLE didn't properly count not null columns for primary keys)
mysql-test/t/alter_table.test:
  More test for bug #6236 (CREATE TABLE didn't properly count not null columns for primary keys)
sql/handler.h:
  Add counter for null fields
sql/sql_table.cc:
  Change create_field->offset to store offset from start of fields, independent of null bits.
  Count null_bits separately from field offsets and adjust them in case of primary key parts.
sql/unireg.cc:
  Change create_field->offset to store offset from start of fields, independent of null bits.
  Count null_bits separately from field offsets and adjust them in case of primary key parts.