maint: use adaptive approach for ulimit -v based tests

When configured with either 'symlinks' or 'shebangs' as value for
the --enable-single-binary option, tests based on `ulimit -v` are
skipped.  The reason is that the multicall 'coreutils' binary requires
much more memory due to shared libraries being loaded, and the size of
the 'date' binary (~290KiB) compared to the multicall binary (~5MiB),
of course.  Finally, in the case of 'shebangs', the starting shell
requires more memory, too

Instead of using hard-coded values for the memory limit, use an
adaptive approach: first determine the amount of memory for a similar,
yet more trivial invocation of the command, and then do the real test
run using that limit (plus some buffer in some cases).

* init.cfg (require_ulimit_v_): Remove function.
(get_min_ulimit_v_): Add function to determine the minimum memory limit
required for a given command in an adaptive way.
* cfg.mk (sc_prohibit_test_ulimit_without_require_): Change the name
of the above function in the syntax-check rule.
* tests/cp/link-heap.sh: Use the above function to determine the
minimum memory required to run a command simpler than in the real test
run.  Use that limit plus a buffer there.  While at it, change to list
of commands in the subshell to fail also if the beginning `ulimit -v`
fails.
* tests/dd/no-allocate.sh: Likewise.
* tests/misc/csplit-heap.sh: Likewise.
* tests/misc/cut-huge-range.sh: Likewise.
* tests/misc/head-c.sh: Likewise.
* tests/misc/printf-surprise.sh: Likewise.
* tests/split/line-bytes.sh: Likewise.
* tests/rm/many-dir-entries-vs-OOM.sh: Likewise - doing it separately
for each program under test.

init.cfg

@@ -144,24 +144,26 @@ require_openat_support_()
   fi
 }
 
-require_ulimit_v_()
+# Determine the minimum required VM limit to run the given command.
+# Output that value to stdout ... to be used by the caller.
+# Return 0 in case of success, and a non-Zero value otherwise.
+get_min_ulimit_v_()
 {
-  local ulimit_works=yes
-  # Expect to be able to exec a program in 10MiB of virtual memory,
-  # (10MiB is usually plenty, but valgrind-wrapped date requires 19000KiB,
-  # so allow more in that case)
-  # but not in 20KiB.  I chose "date".  It must not be a shell built-in
-  # function, so you can't use echo, printf, true, etc.
-  # Of course, in coreutils, I could use $top_builddir/src/true,
-  # but this should be able to work for other projects, too.
   local vm
-  case $(printenv LD_PRELOAD) in */valgrind/*) vm=22000;; *) vm=10000;; esac
-
-  ( ulimit -v $vm; date ) > /dev/null 2>&1 || ulimit_works=no
-  ( ulimit -v 20;  date ) > /dev/null 2>&1 && ulimit_works=no
-
-  test $ulimit_works = no \
-    && skip_ "this shell lacks ulimit support"
+  for v in $( seq 5000 5000 50000 ); do
+    if ( ulimit -v $v && "$@" ) >/dev/null; then
+      local vm_prev
+      prev_v=$v
+      for v in $( seq $(($prev_v-1000)) -1000 1000 ); do
+        ( ulimit -v $v && "$@" ) >/dev/null \
+          || { echo $prev_v; return 0; }
+        prev_v=$v
+      done
+    fi
+  done
+  # The above did not find a working limit.  Echo a very small number - just
+  # in case the caller does not handle the non-Zero return value.
+  echo 1; return 1
 }
 
 require_readable_root_()