Minor changes to dtoa.c to make it easier to work with.

- Change memory allocation to thread a context structure through
   the code rather than using globals (which would require a
   -lpthread and some locking to become threadsafe).

 - Remove a few configuration options (the "private memory" space,
   and K&R style headers).

 - Prefix exported symbols with jvp_ to prevent libc conflicts.

 - Add jvp_dtoa.h

 - Include netlib's g_fmt.c into dtoa.c

 - Add a few not-strictly-necessary initialisations in order to
   compile cleanly with gcc -Wall

c/jv_dtoa.h

@@ -0,0 +1,20 @@
+
+#define Kmax 7
+
+struct Bigint;
+struct dtoa_context {
+  struct Bigint *freelist[Kmax+1];
+  struct Bigint *p5s;
+};
+
+void jvp_dtoa_context_init(struct dtoa_context* ctx);
+void jvp_dtoa_context_free(struct dtoa_context* ctx);
+
+double jvp_strtod(struct dtoa_context* C, const char* s, char** se);
+
+
+char* jvp_dtoa(struct dtoa_context* C, double dd, int mode, int ndigits, int *decpt, int *sign, char **rve);
+void jvp_freedtoa(struct dtoa_context* C, char *s);
+
+#define JVP_DTOA_FMT_MAX_LEN 32
+char* jvp_dtoa_fmt(struct dtoa_context* C, register char *b, double x);