util: Replace non-threadsafe strerror

Some uses of non-threadsafe `strerror` have snuck into the code since
they were removed in #4152. Add a wrapper `SysErrorString` for
thread-safe strerror alternatives and replace all uses of `strerror`
with this.

src/util/sock.cpp

@@ -7,6 +7,7 @@
 #include <threadinterrupt.h>
 #include <tinyformat.h>
 #include <util/sock.h>
+#include <util/syserror.h>
 #include <util/system.h>
 #include <util/time.h>
 
@@ -344,19 +345,8 @@ std::string NetworkErrorString(int err)
 #else
 std::string NetworkErrorString(int err)
 {
-    char buf[256];
-    buf[0] = 0;
-    /* Too bad there are two incompatible implementations of the
-     * thread-safe strerror. */
-    const char *s;
-#ifdef STRERROR_R_CHAR_P /* GNU variant can return a pointer outside the passed buffer */
-    s = strerror_r(err, buf, sizeof(buf));
-#else /* POSIX variant always returns message in buffer */
-    s = buf;
-    if (strerror_r(err, buf, sizeof(buf)))
-        buf[0] = 0;
-#endif
-    return strprintf("%s (%d)", s, err);
+    // On BSD sockets implementations, NetworkErrorString is the same as SysErrorString.
+    return SysErrorString(err);
 }
 #endif