Nettle: Use PROGRAM_CLEAR_STORAGE.
Made _master_location read-only.
Micro-optimization
Removed ATOMIC_SVALUE, it does not compile and does not work.
Made short string size dependent on size of struct pike_string, aim for 25% wasted space on average
Faster svalue type/subtype setting

The code generated for setting two shorts (on x86/x86_64 and arm at least) is
very sub-optimal, especially with gcc, for some reason:

Old push_int(0):
	movq	Pike_interpreter_pointer(%rip), %rdx
	movq	(%rdx), %rax
	leaq	16(%rax), %rcx
	movq	%rcx, (%rdx)
	xorl	%edx, %edx
	xorl	%ecx, %ecx
	movw	%dx, (%rax)
	movw	%cx, 2(%rax)
	movq	$0, 8(%rax)

New push_int(0):
	movq	Pike_interpreter_pointer(%rip), %rdx
	movq	(%rdx), %rax
	leaq	16(%rax), %rcx
	movq	%rcx, (%rdx)
	movq	$0, (%rax)
	movq	$0, 8(%rax)

Except for the lower number of instructions there is an additional
benefit: The old code triggered a read-modify-write operation on most
modern x86 CPU:s, all to preserve the undefined data between subtype
and the value of the svalue. This could be fixed by changing the type
and subtype to be 32-bit instead of 16-bit, but that is a bigger
change.