Fix NTP poll and precision data types

These fields are 8-bit signed integers, per
https://datatracker.ietf.org/doc/html/rfc5905#page-21 (bottom of the
page).

scapy/layers/ntp.py

@@ -448,8 +448,8 @@ class NTPHeader(NTP):
         BitField("version", 4, 3),
         BitEnumField("mode", 3, 3, _ntp_modes),
         BitField("stratum", 2, 8),
-        BitField("poll", 0xa, 8),
-        BitField("precision", 0, 8),
+        SignedByteField("poll", 0xa),
+        SignedByteField("precision", 0),
         FixedPointField("delay", 0, size=32, frac_bits=16),
         FixedPointField("dispersion", 0, size=32, frac_bits=16),
         ConditionalField(IPField("id", "127.0.0.1"), lambda p: p.stratum > 1),
@@ -1120,7 +1120,7 @@ class NTPInfoSys(Packet):
         ByteField("peer_mode", 0),
         ByteField("leap", 0),
         ByteField("stratum", 0),
-        ByteField("precision", 0),
+        SignedByteField("precision", 0),
         FixedPointField("rootdelay", 0, size=32, frac_bits=16),
         FixedPointField("rootdispersion", 0, size=32, frac_bits=16),
         IPField("refid", 0),

test/scapy/layers/ntp.uts

@@ -582,7 +582,7 @@ assert p.data[0].peer == "127.127.1.0"
 assert p.data[0].peer_mode == 3
 assert p.data[0].leap == 0
 assert p.data[0].stratum == 11
-assert p.data[0].precision == 240
+assert p.data[0].precision == -16
 assert p.data[0].refid == "127.127.1.0"
 
 
@@ -1113,7 +1113,7 @@ assert p.data[0].ifname.startswith(b"lo")
 
 from decimal import Decimal
 
-precision = b"\xec"  # 236
+precision = b"\xec"  # -20
 dispersion = b"\x00\x00\xf2\xce"  # 0.948455810546875
 time_stamp = b"\xe6}gt\x00\x00\x00\x00" # Sat, 16 Jul 2022 16:36:04 +0000
 
@@ -1142,7 +1142,7 @@ assert pkt_1.recv.val == time_stamp, pkt_1.recv.val
 
 time_stamp_hex = 0x00000000e67d6774
 pkt_2 = NTP(
-    precision=236,
+    precision=-20,
     dispersion=Decimal('0.948455810546875'),
     orig=time_stamp_hex,
     sent=time_stamp_hex,