Add max_read to fuse_conn_info

Add max_read to fuse_conn_info

Eventually, this setting should be negotiated in the filesystem's init()
handler (like e.g. max_write). However, this requires corresponding
changes in the FUSE kernel module. In preparation for this (and to allow
a transition period) we already allow (and require) filesystems to set
the value in the init() handler in addition to the mount option.

The end-goal is tracked in issue #91.