Add process_iter(attrs, ad_value) parameters to retrieve multiple info in one shot

[giampaolo]

Right now the designated way to iterate over all process and query them is:

>>> import psutil
>>> for proc in psutil.process_iter():
...     try:
...         pinfo = proc.as_dict(attrs=['pid', 'name'])
...     except psutil.NoSuchProcess:
...         pass
...     else:
...         print(pinfo)
...
{'pid': 1, 'name': 'systemd'}
{'pid': 2, 'name': 'kthreadd'}
{'pid': 3, 'name': 'ksoftirqd/0'}
...

This is a bit verbose though and requires a try/except. Also it's a bit error prone (race condition) as the user may forget to catch NoSuchProcess.
Proposal is to introduce 2 new parameters, attrs and ad_value, which will be passed to Process.as_dict().
When attrs is specified Process.as_dict() is called and the resulting dict is stored as a info attribute which is attached to the returned Process instance:

>>> import psutil
>>> for proc in psutil.process_iter(attrs=['pid', 'name']):
...     print(proc.info)
...
{'pid': 1, 'name': 'systemd'}
{'pid': 2, 'name': 'kthreadd'}
{'pid': 3, 'name': 'ksoftirqd/0'}

To retrieve all process info attrs=[] shall be #passed.
This also introduces the possibility to use list/dict comprehensions:

>>> procs = [p for p in psutil.process_iter(attrs=['pid', 'name', 'username'])]
>>> procs
[<psutil.Process(pid=1, name='systemd') at 140154039741328>,
 <psutil.Process(pid=2, name='kthreadd') at 140154039742416>,
 <psutil.Process(pid=3, name='ksoftirqd/0') at 140154039804560>,
 ...]
>>>
>>> info = dict([(p.pid, p.info) for p in procs])
>>> info
{1: {'name': 'systemd', 'pid': 1, 'username': 'root'},
 2: {'name': 'kthreadd', 'pid': 2, 'username': 'root'},
 3: {'name': 'ksoftirqd/0', 'pid': 3, 'username': 'root'},
 ...}