Prefer getsockopt to getpeername on non-MS-Windows

* admin/CPP-DEFINES: Remove HAVE_GETPEERNAME.
* configure.ac: Don’t check for getpeername.
* src/process.c (wait_reading_process_output) [!WINDOWSNT]:
Use getsockopt even if not GNU/Linux, as the platforms where
getsockopt used to hang are no doubt long dead.

admin/CPP-DEFINES

@@ -156,7 +156,6 @@ HAVE_GETLOADAVG
 HAVE_GETOPT_H
 HAVE_GETOPT_LONG_ONLY
 HAVE_GETPAGESIZE
-HAVE_GETPEERNAME
 HAVE_GETPT
 HAVE_GETPWENT
 HAVE_GETRLIMIT

configure.ac

@@ -3848,7 +3848,7 @@ lrand48 random rint \
 select getpagesize setlocale newlocale \
 getrlimit setrlimit shutdown \
 pthread_sigmask strsignal setitimer \
-sendto recvfrom getsockname getpeername getifaddrs freeifaddrs \
+sendto recvfrom getsockname getifaddrs freeifaddrs \
 gai_strerror sync \
 getpwent endpwent getgrent endgrent \
 cfmakeraw cfsetspeed copysign __executable_start log2)

src/process.c

@@ -5492,15 +5492,16 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
 
 	      p = XPROCESS (proc);
 
-#ifdef GNU_LINUX
-	      /* getsockopt(,,SO_ERROR,,) is said to hang on some systems.
-		 So only use it on systems where it is known to work.  */
+#ifndef WINDOWSNT
 	      {
 		socklen_t xlen = sizeof (xerrno);
 		if (getsockopt (channel, SOL_SOCKET, SO_ERROR, &xerrno, &xlen))
 		  xerrno = errno;
 	      }
 #else
+	      /* On MS-Windows, getsockopt clears the error for the
+		 entire process, which may not be the right thing; see
+		 w32.c.  Use getpeername instead.  */
 	      {
 		struct sockaddr pname;
 		socklen_t pnamelen = sizeof (pname);