Portability fixes; required for musl libc.

--- a/telnet/commands.c
+++ b/telnet/commands.c
@@ -29,6 +29,7 @@
 
 #include "telnet_locl.h"
 #include <err.h>
+#include <unistd.h>
 
 #if	defined(IPPROTO_IP) && defined(IP_TOS)
 int tos = -1;
--- a/telnet/ring.h
+++ b/telnet/ring.h
@@ -28,7 +28,8 @@
  *
  */
 
-#include <sys/cdefs.h>
+#include <sys/types.h>
+#define __P(x) x
 #define P __P
 
 /*
--- a/telnet/misc-proto.h
+++ b/telnet/misc-proto.h
@@ -63,7 +63,7 @@
 #ifndef	__MISC_PROTO__
 #define	__MISC_PROTO__
 
-#include <sys/cdefs.h>
+#define __P(x) x
 
 void auth_encrypt_init __P((char *, char *, char *, int));
 void auth_encrypt_user __P((char *));
--- a/telnetd/setproctitle.c
+++ b/telnetd/setproctitle.c
@@ -72,6 +72,9 @@
 static char **Argv = NULL;		/* pointer to argument vector */
 static char *LastArgv = NULL;		/* end of argv */
 static char Argv0[128];			/* program name */
+extern char **environ;
+#undef __environ
+#define __environ environ
 
 void
 initsetproctitle(int argc, char **argv, char **envp)