WL#2936

Fix compiler warnings,
  Fix help output - this fixes im test failures.
  Fix incomplete change of SET plugin vars to ulonglong.
  Allow ER() to work without crashing when errmsg.sys has not been loaded.


include/mysql/plugin.h:
  wl2936
    slip in const modifier for default values,
    this removes compiler warnings when assigning a string const as default value.
sql/derror.cc:
  WL2936
    Allow init_errmessage() to return upon failure.
    Initialize errmesg to an array of empty strings if it failed to load errmsg.sys
sql/mysqld.cc:
  wl2936
    Include Ingo's compiler-warnings fix.
    If init_errmessage() failed to load errmsg.sys, abort.
    Failure to set working directory not fatal when '--help' is specified, as
    server will terminate anyway after displaying help information.
sql/sql_plugin.cc:
  wl2936
    complete change of SET vars from ulong to ulonglong.

include/mysql/plugin.h

@@ -193,7 +193,8 @@ typedef void (*mysql_var_update_func)(MYSQL_THD thd,
 
 #define DECLARE_MYSQL_SYSVAR_BASIC(name, type) struct { \
   MYSQL_PLUGIN_VAR_HEADER;      \
-  type *value; type def_val;    \
+  type *value;                  \
+  const type def_val;           \
 } MYSQL_SYSVAR_NAME(name)
 
 #define DECLARE_MYSQL_SYSVAR_SIMPLE(name, type) struct { \
@@ -215,7 +216,7 @@ typedef void (*mysql_var_update_func)(MYSQL_THD thd,
 #define DECLARE_MYSQL_THDVAR_BASIC(name, type) struct { \
   MYSQL_PLUGIN_VAR_HEADER;      \
   int offset;                   \
-  type def_val;                 \
+  const type def_val;           \
   DECLARE_THDVAR_FUNC(type);    \
 } MYSQL_SYSVAR_NAME(name)