merge revision(s) 45021,45022,45028: [Backport #9524]

	* gc.c: introduce new environment variable
	  "RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR" to control major/minor GC
	  frequency.
	  Do full GC when the number of old objects is more than R * N
	  where R is this factor and

	* test/ruby/test_gc.rb: add a test.

	* gc.c (get_envparam_double): fix a warning message.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@45044 b2dd03c8-39d4-4d8f-98ff-823fe69b080e