all #defined macros in the public headers are considered to be part
of the API and I've generated individual man pages for each of them
to A) make it easier to figure out what each function/macro actually
is for so that automated lookups work better and for B) make sure we
have all public functions document (both macros and functions) to
make it easier for us to work away from all the macros in a future
release.
Functions that return an "int", and friends, return LIBSSH2SFTP_EAGAIN.
Functions that return a structure return NULL and set the error to
LIBSSH2_ERROR_EAGAIN
