Add some missing checks for EINTR after select(2).

I noticed today that Unix Plink responds to SIGWINCH by accidentally
dying of EINTR having interrupted its main select loop, and when I
checked, there turn out to be a couple of other select loops with the
same bug.

unix/uxcons.c

@@ -101,7 +101,9 @@ static int block_and_read(int fd, void *buf, size_t len)
         fd_set rfds;
         FD_ZERO(&rfds);
         FD_SET(fd, &rfds);
-        ret = select(fd+1, &rfds, NULL, NULL, NULL);
+        do {
+            ret = select(fd+1, &rfds, NULL, NULL, NULL);
+        } while (ret < 0 && errno == EINTR);
         assert(ret != 0);
         if (ret < 0)
             return ret;

unix/uxplink.c

@@ -1122,6 +1122,9 @@ int main(int argc, char **argv)
             ret = select(maxfd, &rset, &wset, &xset, NULL);
         }
 
+        if (ret < 0 && errno == EINTR)
+            continue;
+
 	if (ret < 0) {
 	    perror("select");
 	    exit(1);

unix/uxsftp.c

@@ -531,7 +531,9 @@ static int ssh_sftp_do_select(int include_stdin, int no_fds_ok)
                     now = GETTICKCOUNT();
             } while (ret < 0 && errno == EINTR);
         } else {
-            ret = select(maxfd, &rset, &wset, &xset, NULL);
+            do {
+                ret = select(maxfd, &rset, &wset, &xset, NULL);
+            } while (ret < 0 && errno == EINTR);
         }
     } while (ret == 0);