Use vmstate for memory leak regress tests (re: ad9a9219)

'ps' does not always give reliable results; on macOS, 'ps' appears
to produce nondeterministic (i.e. randomly varying) results for
'vsz' and 'rss', making it unusable for memory leak tests. See:
https://github.com/ksh93/ksh/pull/64#issuecomment-655094931
and further comments.

So let's compile in the vmstate builtin so that we can make sure to
measure things properly. It also reports bytes instead of 1024-byte
blocks, so smaller leaks can be detected.

To be decided: whether or not to disable the vmstate builtin for
release builds in order to save about 12K in the ksh binary.

src/cmd/ksh93/data/builtins.c:
- Add vmstate to the list of builtins that are compiled in.

src/cmd/ksh93/tests/leaks.sh:
- getmem(): get size using: vmstate --format='%(busy_size)u'
  (Using busy_size instead of size seems to make more sense as it
  excludes freed blocks. See vmstate --man)
- Introduce a $unit variable for reporting leaks and set it to
  'bytes'; this makes it easier to change the unit in future.
- Since the tests are now more sensitive, initialise all variables
  before use to avoid false leak detections.
- The last test seemed to need a few more 'read' invocations in
  order to get memory usage to a steady state before the test.

src/cmd/ksh93/data/builtins.c

@@ -148,6 +148,7 @@ const struct shtable3 shtab_builtins[] =
 	CMDLIST(uname)
 	CMDLIST(wc)
 	CMDLIST(sync)
+	CMDLIST(vmstate)
 #endif
 #if SHOPT_REGRESS
 	"__regress__",		NV_BLTIN|BLT_ENV,	bltin(__regress__),