strict aliasing violations.

Essentially, the problem is that large parts of the server were
developed in simpler times (last decades, pre C99 standard) when
strict aliasing and compilers supporting such optimizations were
rare to non-existent. Thus, when compiling the server with a modern
compiler that uses strict aliasing rules to perform optimizations,
there are several places in the code that might trigger undefined
behavior.

As evinced by some recent bugs, GCC does a somewhat good of job
misoptimizing such code, but on the other hand also gives warnings
about suspicious code. One problem is that the warnings aren't
always accurate, yet we can't afford to just shut them off as we
might miss real cases. False-positive cases are aggravated mostly
by casts that are likely to trigger undefined behavior.

The solution is to start a cleanup process focused on fixing and
reducing the amount of strict-aliasing related warnings produced
by GCC and others compilers. A good deal of noise reduction can
be achieved by just removing useless casts that are product of
historical cruft and are likely to trigger undefined behavior if
dereferenced.

client/mysql.cc:
  Remove now-unnecessary casts.
  Break up large strings.
client/mysql_upgrade.c:
  Remove now-unnecessary casts.
client/mysqladmin.cc:
  Remove now-unnecessary casts.
  Break up large strings.
client/mysqlbinlog.cc:
  Remove now-unnecessary casts.
client/mysqlcheck.c:
  Remove now-unnecessary casts.
client/mysqldump.c:
  Remove now-unnecessary casts.
client/mysqlimport.c:
  Remove now-unnecessary casts.
client/mysqlshow.c:
  Remove now-unnecessary casts.
client/mysqlslap.c:
  Remove now-unnecessary casts.
client/mysqltest.cc:
  Remove now-unnecessary casts.
extra/comp_err.c:
  Remove now-unnecessary casts.
extra/my_print_defaults.c:
  Remove now-unnecessary casts.
  Break up large strings.
extra/mysql_waitpid.c:
  Remove now-unnecessary casts.
extra/perror.c:
  Remove now-unnecessary casts.
extra/resolve_stack_dump.c:
  Remove now-unnecessary casts.
extra/resolveip.c:
  Remove now-unnecessary casts.
include/my_getopt.h:
  Use a void pointer type as the opaque type to avoid problems with type
  incompatibility -- GCC issues warnings when the type name is not type
  compatible with a operand. As a side bonus, a explicit cast won't be
  necessary anymore.
include/sslopt-longopts.h:
  Remove now-unnecessary casts.
  Break up large strings.
mysys/my_getopt.c:
  Update opaque type and introduce a type definition for the
  argument to my_getopt_register_get_addr.
server-tools/instance-manager/options.cc:
  Remove now-unnecessary casts.
sql/mysqld.cc:
  Remove now-unnecessary casts.
  Break up large strings.
  Update mysql_getopt_value prototype (the old prototype
  was different from the definition anyway).
sql/sql_plugin.cc:
  The type of a pointer to a function must be compatible with the
  pointed-to function type, otherwise the behavior is undefined.
sql/table.cc:
  The variable buf pointer to pointer to pointer to constant char
  could improperly alias a incompatible type in call to fix_type_
  pointers. Since this was actually dead code, it is simply removed.
sql/unireg.cc:
  Remove call to get_form_pos. The code creates a new FRM file which
  is always truncated and writes the form position as 0. Hence, no
  need to retrieve it, we now for sure it is 0.
storage/archive/archive_reader.c:
  Remove now-unnecessary casts.
storage/myisam/ft_nlq_search.c:
  Read weight directly from the buffer.
storage/myisam/fulltext.h:
  Add explanation about the type duality of a key buffer.
  Add accessor macro to retrieve a FT float value.
storage/myisam/mi_test1.c:
  Remove now-unnecessary casts.
storage/myisam/myisam_ftdump.c:
  Read weight directly from the buffer.
storage/myisam/myisamchk.c:
  Remove now-unnecessary casts.
storage/myisam/myisamlog.c:
  A pointer to char was used to alias a pointer to pointer to
  unsigned char, thus violating strict aliasing rules.
storage/myisam/myisampack.c:
  Remove now-unnecessary casts.
strings/decimal.c:
  Remove aliasing violation, printing the value is enough for
  debugging purposes.
tests/mysql_client_test.c:
  Remove now-unnecessary casts.