zstd: Implement core detection (#2083)

The bsdtar manual page claims that setting zstd:threads to 0 tells zstd
to use as many threads as there are cores in the system, but it actually
disables multi-threading.  Replace 0 with the number of configured
processors.

While here, add a previously missing overflow check.

Co-authored-by: Martin Matuska <martin@matuska.de>

build/cmake/config.h.in

@@ -1094,6 +1094,9 @@ typedef uint64_t uintmax_t;
 /* Define to 1 if you have the `symlink' function. */
 #cmakedefine HAVE_SYMLINK 1
 
+/* Define to 1 if you have the `sysconf' function. */
+#cmakedefine HAVE_SYSCONF 1
+
 /* Define to 1 if you have the <sys/acl.h> header file. */
 #cmakedefine HAVE_SYS_ACL_H 1