Add four extra fields to gc_status()

[tstarling]

Exposing gc_active helped me to understand #9323. I thought I may as well just add all the GC globals.

Documentation for new fields:

• running: true if garbage collection is currently running
• protected: true if the garbage collector is protected and root additions are forbidden
• full: true if the garbage collector buffer size exceeds GC_MAX_BUF_SIZE
• buffer_size: current garbage collector buffer size

Documentation for existing fields:

• runs: the number of times the garbage collector has been run
• collected: the number of objects collected
• threshold: the number of roots in the buffer which will trigger garbage collection
• roots: the current number of roots in the buffer

Updated manual example output:

array(8) {
  ["running"]=>
  bool(false)
  ["protected"]=>
  bool(false)
  ["full"]=>
  bool(false)
  ["runs"]=>
  int(5)
  ["collected"]=>
  int(100002)
  ["threshold"]=>
  int(50001)
  ["buffer_size"]=>
  int(131072)
  ["roots"]=>
  int(0)
}

[dstogov]

I don't see any problems.