global: avoid using sprintf.

global: avoid using sprintf.

snprintf has different footguns, but MacOS started complaining, a-la:

Core Lightning fails to compile on macOS with the latest Command Line Tools (macOS 15 SDK) due to sprintf being marked as deprecated and -Werror treating the deprecation as a compilation error.

Environment: Sequoia 15.6.1 (24G90)
SDK Version: MacOSX15.sdk

```
ccan/ccan/closefrom/closefrom.c:83:2: error: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Werror,-Wdeprecated-declarations]
   83 |         sprintf(dnam, "/proc/%ld/fd", (long) getpid());
      |         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_stdio.h:278:1: note: 'sprintf' has been explicitly marked deprecated here
  278 | deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
      | ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:218:48: note: expanded from macro 'deprecated_msg'
  218 |         #define deprecated_msg(_msg) attribute((deprecated(_msg)))
      |                                                       ^
ccan/ccan/closefrom/closefrom.c:162:2: error: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Werror,-Wdeprecated-declarations]
  162 |         sprintf(dnam, "/proc/%ld/fd", (long) getpid());
      |         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/_stdio.h:278:1: note: 'sprintf' has been explicitly marked deprecated here
  278 | deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
      | ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:218:48: note: expanded from macro 'deprecated_msg'
  218 |         #define deprecated_msg(_msg) attribute((deprecated(_msg)))
      |                                                       ^
2 errors generated.
make: *** [ccan-closefrom.o] Error 1
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>