Configurable session read timeout (#892)

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

docs/libssh2_session_get_read_timeout.3

@@ -0,0 +1,19 @@
+.TH libssh2_session_get_read_timeout 3 "13 Jan 2023" "libssh2 1.10.1" "libssh2 manual"
+.SH NAME
+libssh2_session_get_read_timeout - get the timeout for packet read functions
+.SH SYNOPSIS
+#include <libssh2.h>
+.nf
+long libssh2_session_get_read_timeout(LIBSSH2_SESSION *session);
+.SH DESCRIPTION
+Returns the \fBtimeout\fP (in seconds) for how long the ssh2 packet receive
+function calls may wait until they consider the situation an error and
+return LIBSSH2_ERROR_TIMEOUT.
+
+By default the timeout is 60 seconds.
+.SH RETURN VALUE
+The value of the timeout setting.
+.SH AVAILABILITY
+Added in 1.10.1
+.SH SEE ALSO
+.BR libssh2_session_set_read_timeout(3)