strtod from mit-threads is restored and cleaned up


BitKeeper/deleted/.del-atof.c~d3edf47a9884080:
  Delete: strings/atof.c
configure.in:
  atod() is no longer used in MySQL
  isinf() now is
include/m_string.h:
  my_strtod, my_atof
include/my_global.h:
  my_atof is deleted
  define isinf()
libmysql/Makefile.shared:
  use internal strtod
sql/gstream.cc:
  use internal strtod
sql/init.cc:
  my_atof is deleted
sql/item.h:
  use internal strtod
sql/item_func.cc:
  use internal strtod
sql/item_sum.h:
  use internal strtod
sql/sql_analyse.cc:
  use internal strtod
strings/Makefile.am:
  use internal strtod
strings/ctype-simple.c:
  use internal strtod
strings/ctype-ucs2.c:
  use internal strtod
strings/strtod.c:
  cleanup
  stricter input checks (e.g. ".E10" is no longer a number)
  don't return an "inf"