Include <string.h> for some prototypes, rather than depending on

pollution from <strings.h>.

libexec/revnetgroup/parse_netgroup.c

@@ -47,6 +47,7 @@ static const char rcsid[] =
  */
 
 #include <stdio.h>
+#include <string.h>
 #include <strings.h>
 #include <stdlib.h>
 #include <unistd.h>

usr.sbin/arp/arp.c

@@ -78,6 +78,7 @@ static const char rcsid[] =
 #include <paths.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <strings.h>
 #include <unistd.h>
 

usr.sbin/fdformat/fdformat.c

@@ -37,6 +37,7 @@
 #include <paths.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <strings.h>
 #include <sysexits.h>
 #include <unistd.h>