Add Linux's signalfd() to the signal module

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-12-29.01:27:20.548>
labels = ['3.7', '3.8', 'type-feature', 'library']
title = "Add Linux's signalfd() to the signal module"
updated_at = <Date 2018-01-06.07:38:43.164>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2018-01-06.07:38:43.164>
actor = 'njs'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-12-29.01:27:20.548>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 32443
keywords = []
message_count = 6.0
messages = ['309151', '309152', '309166', '309167', '309196', '309229']
nosy_count = 3.0
nosy_names = ['gregory.p.smith', 'pitrou', 'njs']
pr_nums = []
priority = 'low'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32443'
versions = ['Python 3.7', 'Python 3.8']