In case we want to send a lot of data,
and the receiver is slower than the sender.
This will first fill up the receivers queues and after this
eventually also the senders queues,
until the socket is temporarily unable to accept more data to send.
select_write is done with an timeout of zero,
which makes the select call used always return immediately:
(see http://man7.org/linux/man-pages/man2/select.2.html)
This means that every marginal unavailability will make it return false
for is_writable and therefore httplib will immediately abort the transfer.
Therefore make this values configurable in the same way
as the read timeout already is.
Set the default write timeout to 5 seconds,
the same default value used for the read timeout.
According to RFC 3493 the socket option IPV6_V6ONLY
should be off by default, see
https://tools.ietf.org/html/rfc3493#page-22 (chapter 5.3).
However this does not seem to be the case on all systems.
For instance on any Windows OS, the option is on by default.
Therefore clear this option in order to allow
an server socket which can support IPv6 and IPv4 at the same time.
We cannot trivially support such large chunks, and the maximum value
std::strtoul can parse accurately is ULONG_MAX-1. Error out early if the
length is longer than that.
detail::read_content_chunked was using std::stoul to parse the
hexadecimal chunk lengths for "Transfer-Encoding: chunked" requests.
This throws an exception if the string does not begin with any valid
digits. read_content_chunked is not called in the context of a try block
so this caused the process to terminate.
Rather than use exceptions, I opted for std::stroul, which is similar to
std::stoul but does not throw exceptions. Since malformed user input is
not particularly exceptional, and some projects are compiled without
exception support, this approach seems both more portable and more
correct.
