def: add a "high" limit for RLIMIT_NOFILE

This simply adds a new constant we can use for bumping RLIMIT_NOFILE to
a "high" value. It default to 256K for now, which is pretty high, but
smaller than the kernel built-in limit of 1M.

Previously, some tools that needed a higher RLIMIT_NOFILE bumped it to
16K. This new define goes substantially higher than this, following the
discussion with the kernel folks.

src/basic/def.h

@@ -75,3 +75,5 @@
                 _CONF_PATHS_SPLIT_USR(n))
 
 #define LONG_LINE_MAX (1U*1024U*1024U)
+
+#define HIGH_RLIMIT_NOFILE (256*1024)