Linux: Fix propagation of getprotobyname_r() errors.

pcap_nametoproto(), when Linux getprotobyname_r() fails, returns 0 to
fail quickly similarly to pcap_nametonetaddr() and pcap_nametoport().
However, the correct way for pcap_nametoproto() to fail is to return
PROTO_UNDEF.  This is why on a Linux host with GNU libc and without the
/etc/protocols file pcap_nametoproto() and subsequently lookup_proto()
fail to fail, then pcap_compile() produces incorrect filter programs for
"proto NAME", "protochain NAME" and variations thereof:

checking whether getprotobyname_r is declared... yes
checking for the Linux getprotobyname_r()... yes
checking if features.h defines __GLIBC__... yes

@@ -1,6 +1,6 @@
 (000) ldh      [12]
 (001) jeq      #0x800           jt 2	jf 5
 (002) ldb      [23]
-(003) jeq      #0x6             jt 4	jf 5
+(003) jeq      #0x0             jt 4	jf 5
 (004) ret      #262144
 (005) ret      #0

Return the correct error value to fix this and discuss the outputs of
GNU getprotobyname_r() in a comment.

This bug manifested in an s390x Debian 12 container provided by OSU OSL.

Author: infrastation

CHANGES

@@ -104,6 +104,7 @@ DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
       Use getprotobyname_r() correctly on OpenBSD.
     Linux:
       Do not use ether_hostton() from musl libc.
+      Fix propagation of getprotobyname_r() errors.
     Haiku:
       Look for ethers(5) in /boot/system/settings/network/.
     DAG:

nametoaddr.c

@@ -491,12 +491,26 @@ pcap_nametoproto(const char *str)
 	int err;
 
 	err = getprotobyname_r(str, &result_buf, buf, sizeof buf, &p);
+	/*
+	 * As far as GNU libc implementation goes, an "error" means the
+	 * protocol database could not be searched, which could mean err ==
+	 * ERANGE if the buffer is too small or ENOENT if the protocols(5)
+	 * file does not exist (the man page does not document the latter
+	 * eventuality).  If the database has been searched normally and the
+	 * requested protocol name was not found, it is not an "error" and
+	 * err == 0.
+	 *
+	 * This notwithstanding, p == NULL iff a record was not found for any
+	 * reason (whether an "error" or not), which is the same semantics as
+	 * in every other HAVE_xxxxx branch of this block.  The final check
+	 * after the block will catch that if necessary.
+	 */
 	if (err != 0) {
 		/*
 		 * XXX - dynamically allocate the buffer, and make it
 		 * bigger if we get ERANGE back?
 		 */
-		return 0;
+		return PROTO_UNDEF;
 	}
   #elif defined(HAVE_SOLARIS_GETPROTOBYNAME_R)
 	/*