ftp: use closefrom instead of close(pid)

[Theo Buehler]

This variable reuse has a funny smell.

Index: usr.bin/ftp/cmds.c
===================================================================
RCS file: /var/cvs/src/usr.bin/ftp/cmds.c,v
retrieving revision 1.79
diff -u -p -r1.79 cmds.c
--- usr.bin/ftp/cmds.c  21 Jan 2017 08:33:07 -0000      1.79
+++ usr.bin/ftp/cmds.c  23 Jan 2018 08:40:32 -0000
@@ -987,8 +987,7 @@ shell(int argc, char *argv[])
        old1 = signal (SIGINT, SIG_IGN);
        old2 = signal (SIGQUIT, SIG_IGN);
        if ((pid = fork()) == 0) {
-               for (pid = 3; pid < 20; pid++)
-                       (void)close(pid);
+               (void)closefrom(3);
                (void)signal(SIGINT, SIG_DFL);
                (void)signal(SIGQUIT, SIG_DFL);
                shellp = getenv("SHELL");