p_snprintf: no need for arguments to a format

`snprintf` requires a _format_ but does not require _arguments_ to the
format.  eg: `snprintf(buf, 42, "hi")` is perfectly legal.  Expand the
macro to match.

Without this, `p_sprintf(buf, 42, "hi")` errors with:

```
error: expected expression
                p_snprintf(msg, 42, "hi");
                ^
src/unix/posix.h:53:34: note: expanded from macro 'p_snprintf'
                                 ^
/usr/include/secure/_stdio.h:57:73: note: expanded from macro 'snprintf'
  __builtin___snprintf_chk (str, len, 0, __darwin_obsz(str),
__VA_ARGS__)
```

src/unix/posix.h

@@ -50,7 +50,7 @@ extern char *p_realpath(const char *, char *);
 #define p_strcasecmp(s1, s2) strcasecmp(s1, s2)
 #define p_strncasecmp(s1, s2, c) strncasecmp(s1, s2, c)
 #define p_vsnprintf(b, c, f, a) vsnprintf(b, c, f, a)
-#define p_snprintf(b, c, f, ...) snprintf(b, c, f, __VA_ARGS__)
+#define p_snprintf(b, c, ...) snprintf(b, c, __VA_ARGS__)
 #define p_mkstemp(p) mkstemp(p)
 #define p_chdir(p) chdir(p)
 #define p_chmod(p,m) chmod(p, m)