top(1): use modern interfaces for nice and related

- attempt and fail, rather than check for permission.
- use macro rather than explicit "-20"

usr.bin/top/commands.c

@@ -379,14 +379,10 @@ kill_procs(char *str)
     char *nptr;
     int signum = SIGTERM;	/* default */
     int procnum;
-    int uid;
 
     /* reset error array */
     ERR_RESET;
 
-    /* remember our uid */
-    uid = getuid();
-
     /* skip over leading white space */
     while (isspace(*str)) str++;
 
@@ -429,13 +425,8 @@ kill_procs(char *str)
 	}
 	else
 	{
-	    /* check process owner if we're not root */
-	    if (uid && (uid != proc_owner(procnum)))
-	    {
-		ERROR(str, EACCES);
-	    }
 	    /* go in for the kill */
-	    else if (kill(procnum, signum) == -1)
+	    if (kill(procnum, signum) == -1)
 	    {
 		/* chalk up an error */
 		ERROR(str, errno);
@@ -458,10 +449,8 @@ renice_procs(char *str)
     char negate;
     int prio;
     int procnum;
-    int uid;
 
     ERR_RESET;
-    uid = getuid();
 
     /* allow for negative priority values */
     if ((negate = (*str == '-')) != 0)
@@ -499,12 +488,7 @@ renice_procs(char *str)
 	    ERROR(str, 0);
 	}
 
-	/* check process owner if we're not root */
-	else if (uid && (uid != proc_owner(procnum)))
-	{
-	    ERROR(str, EACCES);
-	}
-	else if (setpriority(PRIO_PROCESS, procnum, prio) == -1)
+	if (setpriority(PRIO_PROCESS, procnum, prio) == -1)
 	{
 	    ERROR(str, errno);
 	}

usr.bin/top/top.c

@@ -16,9 +16,9 @@
 #include <sys/time.h>
 #include <sys/cdefs.h>
 #include <sys/limits.h>
+#include <sys/resource.h>
 #include <sys/select.h>
 #include <sys/signal.h>
-#include <time.h>
 
 #include <errno.h>
 #include <getopt.h>
@@ -439,19 +439,13 @@ _Static_assert(sizeof(command_chars) == CMD_toggletid + 2, "command chars size")
 		break;
 
 	      case 'q':		/* be quick about it */
-		/* only allow this if user is really root */
-		if (getuid() == 0)
-		{
-		    /* be very un-nice! */
-		    nice(-20);
-		}
-		else
-		{
-		    fprintf(stderr,
-			"%s: warning: `-q' option can only be used by root\n",
-			myname);
-		    warnings++;
-		}
+			errno = 0;
+			i = setpriority(PRIO_PROCESS, 0, PRIO_MIN);
+			if (i == -1 && errno != 0) {
+				fprintf(stderr,
+						"%s: warning: `-q' option failed (%m)\n", myname);
+				warnings++;
+			}
 		break;
 
 	      case 'm':		/* select display mode */