add realtime signals to strsignal

the name format RTnn/RTnnn was chosen to minimized bloat while
uniquely identifying the signal.

src/string/strsignal.c

@@ -48,7 +48,7 @@ static const char map[] = {
 	[SIGSYS]    = 31
 };
 
-#define sigmap(x) ((unsigned)(x) >= sizeof map ? 0 : map[(unsigned)(x)])
+#define sigmap(x) ((x) >= sizeof map ? (x) : map[(x)])
 
 #endif
 
@@ -84,14 +84,30 @@ static const char strings[] =
 	"Window changed\0"
 	"I/O possible\0"
 	"Power failure\0"
-	"Bad system call";
+	"Bad system call\0"
+	"RT32"
+	"\0RT33\0RT34\0RT35\0RT36\0RT37\0RT38\0RT39\0RT40"
+	"\0RT41\0RT42\0RT43\0RT44\0RT45\0RT46\0RT47\0RT48"
+	"\0RT49\0RT50\0RT51\0RT52\0RT53\0RT54\0RT55\0RT56"
+	"\0RT57\0RT58\0RT59\0RT60\0RT61\0RT62\0RT63\0RT64"
+#if _NSIG > 65
+	"\0RT65\0RT66\0RT67\0RT68\0RT69\0RT70\0RT71\0RT72"
+	"\0RT73\0RT74\0RT75\0RT76\0RT77\0RT78\0RT79\0RT80"
+	"\0RT81\0RT82\0RT83\0RT84\0RT85\0RT86\0RT87\0RT88"
+	"\0RT89\0RT90\0RT91\0RT92\0RT93\0RT94\0RT95\0RT96"
+	"\0RT97\0RT98\0RT99\0RT100\0RT101\0RT102\0RT103\0RT104"
+	"\0RT105\0RT106\0RT107\0RT108\0RT109\0RT110\0RT111\0RT112"
+	"\0RT113\0RT114\0RT115\0RT116\0RT117\0RT118\0RT119\0RT120"
+	"\0RT121\0RT122\0RT123\0RT124\0RT125\0RT126\0RT127\0RT128"
+#endif
+	"";
 
 char *strsignal(int signum)
 {
 	char *s = (char *)strings;
 
 	signum = sigmap(signum);
-	if ((unsigned)signum - 1 > 31) signum = 0;
+	if (signum - 1U >= _NSIG-1) signum = 0;
 
 	for (; signum--; s++) for (; *s; s++);