This set of changes provides a mechanism to runtime-configure the
previously #define'd timeout for reading packets from a session. The
intention here is to also extend libcurl to be able to use this
interface so that when fetching from sftp servers which are very slow
to return directory listings, connections do not time-out so much.
* Add new field to session to hold configurable read timeout
* Updated `_libssh2_packet_require()`, `_libssh2_packet_requirev()`,
and `sftp_packet_requirev()` to use new field in session structure
* Updated docs for API functions to set/get read timeout field in
session structure
* Updated `libssh2.h` to declare the get/set read timeout functions
Co-authored-by: Jon Axtell <jon.axtell@codethink.co.uk>
Credit: Daniel Silverstone
