Make LDMSD create a PID file only when a path is specified

Without the patch, LDMSD always creates a PID file. It determines the
PID file location in the following order: the command-line option `-r`,
the environment variables `LDMSD_PIDFILE`, and the default path
`/var/run/ldmsd.pid`. Running LDMSD as a non-root user withput
specifying a PID file path always causes a permission-denied error
message because a non-root user cannot access `/var/run` directory.

The patch changes LDMSD so that it only create a PID file when either
the command-line option `-r` or the config command `pid_path` is
specified. Regarding starting LDMSD via `systemd`, `systemd` can be
configured to create a PID file for LDMSD.

ldms/src/ldmsd/ldmsd.c

@@ -2208,38 +2208,24 @@ int main(int argc, char *argv[])
 	if (!foreground) {
 		/* Create pidfile for daemon that usually goes away on exit. */
 		/* user arg, then env, then default to get pidfile name */
-		if (!pidfile) {
-			char *pidpath = getenv("LDMSD_PIDFILE");
-			if (!pidpath) {
-				pidfile = malloc(strlen(LDMSD_PIDFILE_FMT)
-						+ strlen(basename(argv[0]) + 1));
-				if (pidfile)
-					sprintf(pidfile, LDMSD_PIDFILE_FMT, basename(argv[0]));
-			} else {
-				pidfile = strdup(pidpath);
+		if (pidfile) {
+			if( !access( pidfile, F_OK ) ) {
+				ovis_log(NULL, OVIS_LERROR, "Existing pid file named '%s': %s\n",
+					pidfile, "overwritten if writable");
 			}
-			if (!pidfile) {
-				ovis_log(NULL, OVIS_LERROR, "Out of memory\n");
-				av_free(auth_opt);
-				exit(1);
+			FILE *pfile = fopen_perm(pidfile,"w", LDMSD_DEFAULT_FILE_PERM);
+			if (!pfile) {
+				int piderr = errno;
+				ovis_log(NULL, OVIS_LERROR, "Could not open the pid file named '%s': %s\n",
+					pidfile, STRERROR(piderr));
+				free(pidfile);
+				pidfile = NULL;
+			} else {
+				pid_t mypid = getpid();
+				fprintf(pfile,"%ld\n",(long)mypid);
+				fclose(pfile);
 			}
 		}
-		if( !access( pidfile, F_OK ) ) {
-			ovis_log(NULL, OVIS_LERROR, "Existing pid file named '%s': %s\n",
-				pidfile, "overwritten if writable");
-		}
-		FILE *pfile = fopen_perm(pidfile,"w", LDMSD_DEFAULT_FILE_PERM);
-		if (!pfile) {
-			int piderr = errno;
-			ovis_log(NULL, OVIS_LERROR, "Could not open the pid file named '%s': %s\n",
-				pidfile, STRERROR(piderr));
-			free(pidfile);
-			pidfile = NULL;
-		} else {
-			pid_t mypid = getpid();
-			fprintf(pfile,"%ld\n",(long)mypid);
-			fclose(pfile);
-		}
 		if (pidfile && banner) {
 			char *suffix = ".version";
 			bannerfile = malloc(strlen(suffix)+strlen(pidfile)+1);