passt: Add cppcheck target, test, and address resulting warnings

...mostly false positives, but a number of very relevant ones too,
in tcp_get_sndbuf(), tcp_conn_from_tap(), and siphash PREAMBLE().

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

test/build/static_checkers

@@ -11,8 +11,12 @@
 # Copyright (c) 2021 Red Hat GmbH
 # Author: Stefano Brivio <sbrivio@redhat.com>
 
-htools	clang-tidy
+htools	clang-tidy cppcheck
 
 test	Run clang-tidy
 hout	RET make clang-tidy; echo $?
 check	[ __RET__ -eq 0 ]
+
+test	Run cppcheck
+hout	RET make cppcheck; echo $?
+check	[ __RET__ -eq 0 ]