fix useless fuse_init_intr_signal in high-level api

fix useless fuse_init_intr_signal in high-level api

The code `fuse_init_intr_signal` in `_fuse_new_317` is useless
since commit 8ee553dac ("fuse_new(): don't accept options that
don't make sense for end-users") has remove the conf.intr option
for end-users. The conf.intr is always zero (i.e., disabled) here.

Move `fuse_init_intr_signal` after the user-defined `init()` function,
so that conf.intr and conf.intr_signal has been configured by the user.