Removed compiler warnings

extra/libevent/epoll.c:
  Removed compiler warnings
extra/libevent/evbuffer.c:
  Removed compiler warnings
extra/libevent/event.c:
  Removed compiler warnings
extra/libevent/select.c:
  Removed compiler warnings
extra/libevent/signal.c:
  Removed compiler warnings
include/m_ctype.h:
  Define CHARSET_INFO, MY_CHARSET_HANDLER, MY_COLLATION_HANDLER, MY_UNICASE_INFO, MY_UNI_CTYPE and MY_UNI_IDX as const structures.
  Declare that pointers point to const data
include/m_string.h:
  Declare that pointers point to const data
include/my_sys.h:
  Redefine variables and function prototypes
include/mysql.h:
  Declare charset as const
include/mysql.h.pp:
  Declare charset as const
include/mysql/plugin.h:
  Declare charset as const
include/mysql/plugin.h.pp:
  Declare charset as const
mysys/charset-def.c:
  Charset can't be of type CHARSET_INFO as they are changed when they are initialized.
mysys/charset.c:
  Functions that change CHARSET_INFO must use 'struct charset_info_st'
  Add temporary variables to not have to change all_charsets[] (Which now is const)
sql-common/client.c:
  Added cast to const
sql/item_cmpfunc.h:
  Added cast to avoid compiler error.
sql/sql_class.cc:
  Added cast to const
sql/sql_lex.cc:
  Added cast to const
storage/maria/ma_ft_boolean_search.c:
  Added cast to avoid compiler error.
storage/maria/ma_ft_parser.c:
  Added cast to avoid compiler error.
storage/maria/ma_search.c:
  Added cast to const
storage/myisam/ft_boolean_search.c:
  Added cast to avoid compiler error
storage/myisam/ft_parser.c:
  Added cast to avoid compiler error
storage/myisam/mi_search.c:
  Added cast to const
storage/pbxt/src/datadic_xt.cc:
  Added cast to const
storage/pbxt/src/ha_pbxt.cc:
  Added cast to const
  Removed compiler warning by changing prototype of XTThreadPtr()
storage/pbxt/src/myxt_xt.h:
  Character sets should be const
storage/pbxt/src/xt_defs.h:
  Character sets should be const
storage/xtradb/btr/btr0cur.c:
  Removed compiler warning
strings/conf_to_src.c:
  Added const
  Functions that change CHARSET_INFO must use 'struct charset_info_st'
strings/ctype-big5.c:
  Made arrays const
strings/ctype-bin.c:
  Made arrays const
strings/ctype-cp932.c:
  Made arrays const
strings/ctype-czech.c:
  Made arrays const
strings/ctype-euc_kr.c:
  Made arrays const
strings/ctype-eucjpms.c:
  Made arrays const
strings/ctype-extra.c:
  Made arrays const
strings/ctype-gb2312.c:
  Made arrays const
strings/ctype-gbk.c:
  Made arrays const
strings/ctype-latin1.c:
  Made arrays const
strings/ctype-mb.c:
  Made arrays const
strings/ctype-simple.c:
  Made arrays const
strings/ctype-sjis.c:
  Made arrays const
strings/ctype-tis620.c:
  Made arrays const
strings/ctype-uca.c:
  Made arrays const
strings/ctype-ucs2.c:
  Made arrays const
strings/ctype-ujis.c:
  Made arrays const
strings/ctype-utf8.c:
  Made arrays const
strings/ctype-win1250ch.c:
  Made arrays const
strings/ctype.c:
  Made arrays const
  Added cast to const
  Functions that change CHARSET_INFO must use 'struct charset_info_st'
strings/int2str.c:
  Added cast to const