Don't handle --help and --version in fuse_session_new().

Don't handle --help and --version in fuse_session_new().

Help and version messages can be generated using the new
fuse_lowlevel_help(), fuse_lowlevel_version(), fuse_mount_help(), and
fuse_mount_version() functions.

The fuse_parse_cmdline() function has been made more powerful
to do this automatically, and is now explicitly intended only
for low-level API users.

This is a code simplication patch. We don't have to parse for --help and
--version in quite as many places, and we no longer have a low-level
initialization function be responsible for the (super-high level) task
of printing a program usage message.

In the high-level API, we can now handle the command line parsing
earlier and avoid running other initialization code if we're just going
to abort later on.