sysconf: fix _SC_MINSIGSTKSZ computation to match kernel interpretation

the value placed in the aux vector AT_MINSIGSTKSZ by the kernel is
purely the signal frame size, and does not include any execution space
for the signal handler. this is contrary to the POSIX definition of
MINSIGSTKSZ to be a value that can actually execute at least some
minimal signal handler, and contrary to the historical definitions of
MINSIGSTKSZ which had at least 1k of headroom.

src/conf/sysconf.c

@@ -220,8 +220,13 @@ long sysconf(int name)
 		return (mem > LONG_MAX) ? LONG_MAX : mem;
 	case JT_MINSIGSTKSZ & 255:
 	case JT_SIGSTKSZ & 255: ;
-		long val = __getauxval(AT_MINSIGSTKSZ);
-		if (val < MINSIGSTKSZ) val = MINSIGSTKSZ;
+		/* Value from auxv/kernel is only sigfame size. Clamp it
+		 * to at least 1k below arch's traditional MINSIGSTKSZ,
+		 * then add 1k of working space for signal handler. */
+		unsigned long sigframe_sz = __getauxval(AT_MINSIGSTKSZ);
+		if (sigframe_sz < MINSIGSTKSZ - 1024)
+			sigframe_sz = MINSIGSTKSZ - 1024;
+		unsigned val = sigframe_sz + 1024;
 		if (values[name] == JT_SIGSTKSZ)
 			val += SIGSTKSZ - MINSIGSTKSZ;
 		return val;