Bounds-checks and blocksize corrections were applied to user-input,
but constants in the server were trusted implicitly. If these values
did not actually meet the requirements, the user could not set change
a variable, then set it back to the (wonky) factory default or maximum
by explicitly specifying it (SET <var>=<value> vs SET <var>=DEFAULT).

Now checks also apply to the server's presets. Wonky values and maxima
get corrected at startup. Consequently all non-offsetted values the user
sees are valid, and users can set the variable to that exact value if
they so desire.

mysql-test/r/read_buffer_size_basic.result:
  test sets out of bounds value; we now throw a warning for this.
  This is a side-effect: before, the maximum was higher than the
  value we set here. The value was corrected to block-size, the
  maximum was not, hence the value was smaller than the maximum
  in this particular case. Now that we align the maxima at startup,
  the value in SET is larger than the (corrected) maximum, and we
  see a warning in this particular case. "This means we're doing it right."
mysql-test/r/read_rnd_buffer_size_basic.result:
  test sets out of bounds value; we now throw a warning for this.
  This is a side-effect: before, the maximum was higher than the
  value we set here. The value was corrected to block-size, the
  maximum was not, hence the value was smaller than the maximum
  in this particular case. Now that we align the maxima at startup,
  the value in SET is larger than the (corrected) maximum, and we
  see a warning in this particular case. "This means we're doing it right."
mysys/my_getopt.c:
  Do bounds-checking at start-up time so we'll catch and correct
  wonky default values and upper limits.
sql/mysqld.cc:
  If 0 is a legal value per the docs, not to mention the default, we shouldn't give 1 as
  the lower limit.
storage/innobase/handler/ha_innodb.cc:
  We are setting upper bounds here.
  ~0L gives -1. That is NOT what we want!