httpd: return what a command printed over /cmd

POST /cmd runs the command and answers with an empty 200, so a caller cannot
tell "done" from "nothing happened", and whatever the command prints only
reaches the serial console. Reading a register over the network needs a second
channel just to see the answer.

write_char() is the one funnel every console helper goes through, and it already
tees into the syslog buffer. A third sink, live only while the httpd runs a
command that arrived over the network, puts the same bytes into the response
behind the header. Serial and syslog keep getting their copy.

The capture stops at the end of the response buffer, so a long listing is
truncated rather than overrunning it.

BANK1 grows 39 bytes and the common area 52. One byte of xdata, BANK2 unchanged.

rtlplayground.c

@@ -240,6 +240,12 @@ void isr_serial(void) __interrupt(4)
 }
 
 
+/* Set by the httpd while it runs a command that arrived over the network, so
+ * that everything the command prints lands in the response as well. */
+__xdata uint8_t cmd_capture;
+extern __xdata uint8_t outbuf[TCP_OUTBUF_SIZE];
+extern __xdata uint16_t slen;
+
 void write_char_no_syslog(char c)
 {
 	do {
@@ -256,6 +262,9 @@ void write_char_no_syslog(char c)
 
 void write_char(char c)
 {
+	if (cmd_capture && slen < TCP_OUTBUF_SIZE - 1)
+		outbuf[slen++] = c;
+
 	write_char_no_syslog(c);
 
 	if (syslog_state.enabled) {