maint: Remove unnecessary standard library extern function decls.

* ftp/cmds.c (account, user): Don't declare getpass since it is
defined in unistd.h.
* ftp/ftp.c (login): Likewise. Don't declare fclose and pclose since
they are defined in stdio.h.
* ftpd/extern.h: Don't declare getusershell since it is defined in
unistd.h.
* ftpd/ftpd.c: Don't declare fclose since it is defined in stdio.h.
* libinetutils/logwtmp.c: Don't declare errno since it is defined in
errno.h.
* libtelnet/misc.c (auth_encrypt_user): Include string.h and don't
declare strdup.
* telnet/authenc.c (telnet_gets):  Don't declare getpass since it is
defined in unistd.h.
* telnet/commands.c (hostname): Use NULL instead of 0.
(env_init, shell): Include string.h and don't redeclare functions.

libinetutils/logwtmp.c

@@ -44,10 +44,6 @@
 #endif
 #include <string.h>
 
-#if !HAVE_DECL_ERRNO
-extern int errno;
-#endif
-
 #ifdef HAVE_UTMPX_H
 static void
 _logwtmp (struct utmpx *ut)