Also fixes a new bug for which "NULL" wasn't printed (because the 
data it represents has length zero).  (Discovered my Paul DuBois.)


client/mysql.cc:
  Cleaned up the interactive-session table-printing function.
  
  - No longer rely on the length of the data to pad column boundries.
  - Be smarter about how we detect if the column is NULL.
  - Document how multibyte characters affect the output printing.
  - Use more descriptive variable names.
  
  More importantly, (re-)add these features that were crippled in an
  earlier change:
  - Print "NULL".
  - Right-justify numbers.
mysql-test/r/mysql.result:
  Updated old result and added new case.
mysql-test/t/mysql.test:
  Added new test case.