restore dtostrf when floats are disabled in printf/scanf + round fix

[d-a-v]

replaces and closes #7087

(edit: tested)

fixes #7073 with solution 6
updates #7068
fixes #7043
(no need for -DNOPRINTFLOAT)

[earlephilhower]

Looks like the if (function pointer) is not optimized. Both branches are present in all builds (float and non-float). That gives a 554-byte function in both cases. In normal mode, ~500 of those bytes are never used (the __dtostrf inlined code), and in non-float mode ~50 of those aren't used (the printf() branch).

I think this may be the best we can hope for, without adding a new #define like your original PR, so I'm approving as-is. If someone else has any ideas, I'd be interested...

[d-a-v]

I am too interested in another solution because I see only the use of the define to not have the real implementation when printf-float is available. That's because the symbol presence cannot be known at compile time.

@earlephilhower with a patch to newlib, we could condition linking of _prinf_float the very same way we do with stack in sys space when WPS is not used. No more -u printf... nor any define. A simple call to a function could disable printf/floats at link time.

[d-a-v]

diff from original dtostrf is

index b39edbb2..c742904d 100644
--- a/cores/esp8266/core_esp8266_noniso.cpp
+++ b/cores/esp8266/core_esp8266_noniso.cpp
@@ -27,6 +27,7 @@
 #include <stdbool.h>
 #include <stdint.h>
 #include <math.h>
+#include <limits>
 #include "stdlib_noniso.h"
 
 extern "C" {
@@ -77,11 +78,15 @@ char * dtostrf(double number, signed char width, unsigned char prec, char *s) {
     // Figure out how big our number really is
     double tenpow = 1.0;
     int digitcount = 1;
-    while (number >= 10.0 * tenpow) {
-        tenpow *= 10.0;
+    double nextpow;
+    while (number >= (nextpow = (10.0 * tenpow))) {
+        tenpow = nextpow;
         digitcount++;
     }
 
+    // minimal compensation for possible lack of precision (#7087 addition)
+    number *= 1 + std::numeric_limits<decltype(number)>::epsilon();
+
     number /= tenpow;
     fillme -= digitcount;