Avoid max-idle threads warning

Avoid max-idle threads warning

If a program with API before 312 did not set
max_idle_threads the new default from
fuse_parse_cmdline_312() is applied, which sets
UINT_MAX (-1).

Later in compat fuse_session_loop_mt_32 the old
config v1 struct is converted and that conversion
prints a warning if the default unset value was used.

This could have also happened to programs using the current
API, which just apply values struct fuse_cmdline_opts,
without checking if the defaults are set.