ndppd: fix compile error with musl

ndppd: fix compile error with musl

Error:
src/logger.cc: In static member function 'static std::string ndppd::logger::err()':
src/logger.cc:86:22: error: could not convert 'strerror_r((*__errno_location()), ((char*)(& buf)), sizeof (buf))'
                            from 'int' to 'std::string {aka std::basic_string<char>}'
    return strerror_r(errno, buf, sizeof(buf));

Added and modified some patches from
https://github.com/DanielAdolfsson/ndppd/compare/eb81b8f2d6d4d33545570402b049a73880b9ad01...navossoc:master
(See https://github.com/DanielAdolfsson/ndppd/issues/40 for details)

Compile-tested: octeon + mpc85xx openwrt master, musl + uclibc++
                octeon openwrt master, glibc + libstdc++

Run-tested: octeon openwrt master, musl + uclibc++

Signed-off-by: Martin Strobel <arctus@crza.de>