lib: deprecate memset_zero template, use C99 compound literals instead

Sprinkle in some asserts to make sure compilers aren't getting creative.

This may introduce a new compiler dependency, as I suspect older versions
of GCC don't support this syntax.

It definitely needs a new compiler flag to suppress a warning when some
fields are not explicitly initialized.  If we've omitted a field, it's
because it's a field we don't know (or care) about, and we want that
thing initialized to zero.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/fd.cc

@@ -477,9 +477,9 @@ namespace crucible {
 		return pwrite_or_die(fd, text.data(), text.size(), offset);
 	}
 
-	Stat::Stat()
+	Stat::Stat() :
+		stat( (stat) { } )
 	{
-		memset_zero<stat>(this);
 	}
 
 	Stat &
@@ -498,15 +498,15 @@ namespace crucible {
 		return *this;
 	}
 
-	Stat::Stat(int fd)
+	Stat::Stat(int fd) :
+		stat( (stat) { } )
 	{
-		memset_zero<stat>(this);
 		fstat(fd);
 	}
 
-	Stat::Stat(const string &filename)
+	Stat::Stat(const string &filename) :
+		stat( (stat) { } )
 	{
-		memset_zero<stat>(this);
 		lstat(filename);
 	}