Fix various mismatches between function's language linkage. Any
particular function that is declared in C++ but should be callable
from C must have C linkage. Note that function types with different
linkages are also distinct. Thus, if a function type is declared in
C code, it will have C linkage (same if declared in a extern "C"
block).

client/mysql.cc:
  Mismatch between prototype and declaration.
client/mysqltest.cc:
  mysqltest used to be C code. Use C linkage where appropriate.
cmd-line-utils/readline/input.c:
  Isolate unreachable code.
include/my_alloc.h:
  Function type must have C linkage.
include/my_base.h:
  Function type must have C linkage.
include/my_global.h:
  Add helper macros to avoid spurious namespace indentation.
include/mysql.h.pp:
  Update ABI file.
mysys/my_gethwaddr.c:
  Remove stray carriage return and fix coding style.
plugin/semisync/semisync_master_plugin.cc:
  Callback function types have C linkage.
plugin/semisync/semisync_slave_plugin.cc:
  Callback function types have C linkage.
sql/derror.cc:
  Expected function type has C linkage.
sql/field.cc:
  Use helper macro and fix indentation.
sql/handler.cc:
  Expected function type has C linkage.
sql/item_sum.cc:
  Correct function linkages. Remove now unnecessary cast.
sql/item_sum.h:
  Add prototypes with the appropriate linkage as otherwise they
  are distinct.
sql/mysqld.cc:
  Wrap functions in C linkage mode.
sql/opt_range.cc:
  C language linkage is ignored for class member functions.
sql/partition_info.cc:
  Add wrapper functions with C linkage for class member functions.
sql/rpl_utility.h:
  Use helper macro and fix indentation.
sql/sql_class.cc:
  Change type of thd argument -- THD is a class.
  Use helper macro and fix indentation.
sql/sql_class.h:
  Change type of thd argument -- THD is a class.
sql/sql_select.cc:
  Expected function type has C linkage.
sql/sql_select.h:
  Move prototype to sql_test.h
sql/sql_show.cc:
  Expected function type has C linkage.
sql/sql_test.cc:
  Fix required function prototype and fix coding style.
sql/sql_test.h:
  Removed unnecessary export and add another.
storage/myisammrg/ha_myisammrg.cc:
  Expected function type has C linkage.
storage/perfschema/pfs.cc:
  PSI headers are declared with C language linkage, which also
  applies to function types.