Code changes:
1. Expose daemon_poll() so lightningd can call it directly, which avoids us
having store a global and document it.
2. Remove the (undocumented, unused, forgotten) --rpc-file="" option to disable
JSON RPC.
3. Move the ickiness of finding the executable path into subd.c, so it doesn't
distract from lightningd.c overview.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
