fwdmachine: verify upstream TLS certificates

[KernelClint]

ForwardMachine in TLS mode terminates the client's connection and opens its own onward connection
to the real server. The downstream side is an interception by design — it generates lookalike
certificates — but the upstream side is an ordinary TLS client connection.

scapy/fwdmachine.py:365-379
loads the system certificates and then switches both forms of authentication off:

clisslcontext = ssl.SSLContext(ssl.PROTOCOL_TLS)
clisslcontext.load_default_certs()
clisslcontext.check_hostname = False
clisslcontext.verify_mode = ssl.CERT_NONE

Anything on the path between the machine and the origin can therefore present any certificate and
have its data relayed onward. A self-signed upstream and a validly signed one for a different
hostname both went through.

The change uses Python's default client policy, which requires a trusted chain and checks the
hostname:

-            clisslcontext = ssl.SSLContext(ssl.PROTOCOL_TLS)
-            clisslcontext.load_default_certs()
-            clisslcontext.check_hostname = False
-            clisslcontext.verify_mode = ssl.CERT_NONE
+            if self.verify_upstream:
+                clisslcontext = ssl.create_default_context()
+            else:
+                clisslcontext = ssl.SSLContext(ssl.PROTOCOL_TLS)
+                clisslcontext.load_default_certs()
+                clisslcontext.check_hostname = False
+                clisslcontext.verify_mode = ssl.CERT_NONE

Two details matter for not breaking working setups.

The hostname checked comes from the downstream ClientHello's SNI, and a client may send none. The
name passed to wrap_socket() therefore falls back to the destination, matching the idiom the
certificate cache already uses at scapy/fwdmachine.py:349 (ident = server_name or dest):

-                ss = clisslcontext.wrap_socket(ss, server_hostname=server_name)
+                ss = clisslcontext.wrap_socket(
+                    ss, server_hostname=server_name or dest[0]
+                )

And because interception against an upstream that cannot be verified is a legitimate use of this
class, verify_upstream=True is a constructor option rather than a hard change of behaviour.

The added regressions assert the verifying context is used by default, the opt-out still reaches
the old one, and a connection with no SNI is checked against the destination host. Reverting the
source with the tests in place fails them.

Performance was measured on one computer, before and after the fix: building the context took
758.7 ns before and 542.2 ns after — 28.5% faster. Repeat runs moved by about 3%, so this is
larger than the test's own variation. create_default_context() does in one step what the previous
four lines did separately.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.72%. Comparing base (b3bbcc8) to head (d9cb53b).

Files with missing lines	Patch %	Lines
scapy/fwdmachine.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5141      +/-   ##
==========================================
+ Coverage   80.63%   80.72%   +0.09%     
==========================================
  Files         390      390              
  Lines       96936    96939       +3     
==========================================
+ Hits        78168    78258      +90     
+ Misses      18768    18681      -87     

Files with missing lines	Coverage Δ
scapy/fwdmachine.py	31.43% <50.00%> (+31.43%)	⬆️

... and 6 files with indirect coverage changes

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