* gc.c: enable "RGENGC_ESTIMATE_OLDSPACE" option as default.
  Without this option, some application consumes huge memory.
  (and there are only a few performance down)
  Introduced new environment variables:
  * RUBY_GC_HEAP_OLDSPACE (default 16MB)
  * RUBY_GC_HEAP_OLDSPACE_MAX (default 128 MB)
  * RUBY_GC_HEAP_OLDSPACE_GROWTH_FACTOR (default 1.2)
* gc.c (initial_malloc_limit): rename to initial_malloc_limit_min.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43755 b2dd03c8-39d4-4d8f-98ff-823fe69b080e