scst: Use nanosleep for userspace polling delays

Retain the 50 ms and 100 ms polling intervals and the existing
signal-interruption behavior while replacing the obsolete usleep
interface.

Cppcheck reports usleepCalled:

  Obsolescent function 'usleep' called. It is recommended to use
  'nanosleep' or 'setitimer' instead.

iscsi-scst/usr/target.c

@@ -28,6 +28,7 @@
 #include <netdb.h>
 #include <ifaddrs.h>
 #include <unistd.h>
+#include <time.h>
 
 #include "iscsid.h"
 
@@ -376,7 +377,7 @@ int target_del(u32 tid, u32 cookie)
 
 		/* We have not yet received session(s) removal event(s), so keep waiting */
 		log_debug(1, "Target %d has sessions, keep waiting", tid);
-		usleep(50000);
+		nanosleep(&(const struct timespec){ .tv_nsec = 50000000 }, NULL);
 	}
 
 	/*

usr/stpgd/stpgd_main.c

@@ -15,6 +15,7 @@
 #include <stdio.h>
 #include <pthread.h>
 #include <unistd.h>
+#include <time.h>
 #include <syslog.h>
 #include <signal.h>
 #include <syslog.h>
@@ -196,7 +197,7 @@ static int wait_until_finished(pid_t pid, unsigned long deadline, int *status, i
 			}
 			break;
 		}
-		usleep(100*1000);
+		nanosleep(&(const struct timespec){ .tv_nsec = 100000000 }, NULL);
 		time(&end);
 		elapsed = difftime(end, start);
 	} while (elapsed < deadline);