timeval options do not have symmetric set/get #95
Description
Issue description:
Infinite timeout is set using a negative number, but turns into None when retreived.
Setting timeout to None leads to an internal error.
>>> import ldap
>>> conn = ldap.initialize('https://example.com')
>>> conn.set_option(ldap.OPT_NETWORK_TIMEOUT, -1)
>>> print(conn.get_option(ldap.OPT_NETWORK_TIMEOUT))
None
>>> conn.set_option(ldap.OPT_NETWORK_TIMEOUT, None)
Traceback (most recent call last):
...
SystemError: <built-in method set_option of LDAP object at 0x7f943715dd78> returned a result with an error setOperating system: Fedora
Python version: 3.6.3
python-ldap version: 3.0.0b1
Imported from https://sourceforge.net/p/python-ldap/bugs/48/