Fixed problem with `stop` on windows

Changed CPPHTTPLIB_IDLE_INTERVAL_USECOND to 0

Code cleanup and format

Fixed build errors with some examples

Updated version in the User Agent string

Fix yhirose#472

Fixed potential infinite loop with content receiver

Appled HANDLE_EINTR to `send` and `select` system calls

Code format

SSLClient::set_ca_cert_store: mark as inline (yhirose#435)