Incorporate strerror_r() into src/port/snprintf.c, too.

This provides the features that used to exist in useful_strerror()
for users of strerror_r(), too.  Also, standardize on the GNU convention
that strerror_r returns a char pointer that may not be NULL.

I notice that libpq's win32.c contains a variant version of strerror_r
that probably ought to be folded into strerror.c.  But lacking a
Windows environment, I should leave that to somebody else.

Discussion: https://postgr.es/m/2975.1526862605@sss.pgh.pa.us

src/interfaces/libpq/libpq-int.h

@@ -773,7 +773,7 @@ extern char *libpq_ngettext(const char *msgid, const char *msgid_plural, unsigne
 #define SOCK_ERRNO_SET(e) WSASetLastError(e)
 #else
 #define SOCK_ERRNO errno
-#define SOCK_STRERROR pqStrerror
+#define SOCK_STRERROR strerror_r
 #define SOCK_ERRNO_SET(e) (errno = (e))
 #endif