Add SFP quirk for devices that misreport DDM capability

On a QSFPTEK QT-SFP+-T (RTL8261C) 10GBase-T module, the diag type field
(92) comes back as 0x00, but there's actually some statistics available
like temperature. Other metrics may be hard-coded values.

Add a basic struct that allows matching on vendor and/or model, using a
bitfield to allow multiple quirks for a given SFP module. Only
SFP_QUIRK_DDM is implemented.

For modules matching SFP_QUIRK_DDM, attempt an I2C read of the MSB of
module voltage during probe if DDM is "unsupported" - if it's not 0xff,
override the reported options so we can pull the diagnostic data.

To allow simpler comparisons, convert the ASCII fields (vendor,
model, serial) from space-padded to standard NULL-terminated strings.

strcmp() is moved from httpd.c to rtlplayground.c alongside other string
functions and shared between them.

The __reentrant keyword is used for the new functions to avoid using up
additional OSEG space. This allocates the variables on the stack, which
is OK for this particular code path.

The JSON assembly in send_status() is slightly modified to treat the
sfp_module_* data as standard NULL-terminated strings, and a repeated
subtraction was moved into a uint8_t to declutter the code.

rtl837x_common.h

@@ -158,6 +158,7 @@ uint16_t strlen(register __code const char *s);
 uint16_t strlen_x(register __xdata const char *s);
 uint16_t strtox(register __xdata uint8_t *dst, register __code const char *s);
 uint16_t strcpy(register __xdata uint8_t *dst, register const char *s);
+char strcmp(register __xdata const uint8_t *a, register __code const uint8_t *b);
 void tcpip_output(void);
 uint8_t read_flash(uint8_t bank, __code uint8_t *addr);
 void get_random_32(void);