Fix confusing "resuming interrupted unknown" message

Dmitry reported seeing the following:

    $ sleep 2 & sleep 1; ./strace -erestart_syscall -p $!
    [1] 12345
    Process 12345 attached
    restart_syscall(<... resuming interrupted unknown ...>) = 0

Fixing it by replacing "unknown" with "system call".

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/syscall.c b/syscall.c
index d54d8b6..1e19eb5 100644
--- a/syscall.c
+++ b/syscall.c
@@ -1264,7 +1264,7 @@
 			.nargs = MAX_ARGS,
 			.sys_flags = 0,
 			.sys_func = printargs,
-			.sys_name = "unknown", /* not used */
+			.sys_name = "system call",
 		};
 		tcp->s_ent = &unknown;
 		tcp->qual_flg = UNDEFINED_SCNO | QUAL_RAW | DEFAULT_QUAL_FLAGS;