Use NULL instead of 0 for pointers.

getenv(3) returns NULL if the variable name is not in the current
environment.
getservent(3) returns NULL on EOF or error

MFC after:	4 weeks

usr.sbin/ppp/command.c

@@ -651,7 +651,7 @@ ShellCommand(struct cmdargs const *arg, int bg)
   if ((shpid = fork()) == 0) {
     int i, fd;
 
-    if ((shell = getenv("SHELL")) == 0)
+    if ((shell = getenv("SHELL")) == NULL)
       shell = _PATH_BSHELL;
 
     timer_TermService();

usr.sbin/ppp/filter.c

@@ -80,7 +80,7 @@ ParsePort(const char *service, const char *proto)
   int port;
 
   servent = getservbyname(service, proto);
-  if (servent != 0)
+  if (servent != NULL)
     return ntohs(servent->s_port);
 
   port = strtol(service, &cp, 0);