shared/strbuf: don't have const values in NMStrBuf

Before, if a struct had a field of type NMStrBuf (which is sensible to do),
then you could not longer initialize the entire struct with

  *ptr = (Type) { };

because NMStrBuf contained const fields.

The user should never set these fields directly and use nm_str_buf_*() to modify
them them. But no longer mark them as const, because that breaks valid
use cases.

shared/nm-glib-aux/nm-str-buf.h

@@ -13,7 +13,7 @@
 typedef struct _NMStrBuf {
 	char *_str;
 	union {
-		const gsize len;
+		/*const*/ gsize len;
 		gsize _len;
 	};
 	gsize _allocated;