Backport (with some extra nonnull fixes) from trunk to 5.7 of:

  Bug#23195845: FIX COMPILE WARNINGS REPORTED BY GCC 6.1

  Fix compile warnings reported by recently released GCC 6.1:

  -Werror=misleading-indentation
  libbinlogevents/src/uuid.cpp
  rapid/plugin/x/mysqlxtest_src/mysqlx.cc
  sql/sql_update.cc
  This error had to be disabled for files using Boost.

  -Werror=nonnull-compare
  storage/innobase/include/dict0dict.ic

  -Werror=logical-op
  mysys/mf_dirname.cc
  sql/item_timefunc.cc
  sql/mysqld.cc
  vio/viosocket.cc

  Also fix problem in mysys/my_sync.cc where we were using
  __linux rather than the correct __linux__ symbol.

  (cherry picked from commit 43a1c9dca6104a35455721eddeabcc748e349bbb)

Change-Id: I9580f680a4bceceb649bf61e82eac552938690b6