Reduce stack space consumption in tzload().

While syncing our timezone code with IANA's updates in commit 1c1a7cb,
I'd chosen not to adopt the code they conditionally compile under #ifdef
ALL_STATE.  The main thing that that drives is that the space for gmtime
and localtime timezone definitions isn't statically allocated, but is
malloc'd on first use.  I reasoned we didn't need that logic: we don't have
localtime() at all, and we always initialize TimeZone to GMT so we always
need that one.  But there is one other thing ALL_STATE does, which is to
make tzload() malloc its transient workspace instead of just declaring it
as a local variable.  It turns out that that local variable occupies 78K.
Even worse is that, at least for common US timezone settings, there's a
recursive call to parse the "posixrules" zone name, making peak stack
consumption to select a time zone upwards of 150K.  That's an uncomfortably
large fraction of our STACK_DEPTH_SLOP safety margin, and could result in
outright crashes if we try to reduce STACK_DEPTH_SLOP as has been discussed
recently.  Furthermore, this means that the postmaster's peak stack
consumption is several times that of a backend running typical queries
(since, except on Windows, backends inherit the timezone GUC values and
don't ever run this code themselves unless you do SET TIMEZONE).  That's
completely backwards from a safety perspective.

Hence, adopt the ALL_STATE rather than non-ALL_STATE variant of tzload(),
while not changing the other code aspects that symbol controls.  The
risk of an ENOMEM error from malloc() seems less than that of a SIGSEGV
from stack overrun.

This should probably get back-patched along with 1c1a7cb and followon
fixes, whenever we decide we have enough confidence in the updates to do
that.

Author: tglsfdc

src/timezone/localtime.c

@@ -536,9 +536,17 @@ tzloadbody(char const * name, char *canonname, struct state * sp, bool doextend,
 int
 tzload(const char *name, char *canonname, struct state * sp, bool doextend)
 {
-	union local_storage ls;
+	union local_storage *lsp = malloc(sizeof *lsp);
 
-	return tzloadbody(name, canonname, sp, doextend, &ls);
+	if (!lsp)
+		return errno;
+	else
+	{
+		int			err = tzloadbody(name, canonname, sp, doextend, lsp);
+
+		free(lsp);
+		return err;
+	}
 }
 
 static bool