Spider tweaks

Reworks the spider "fur" to something marginally more realistic, links the chitin and fur branches, and adjusts a dream now that spiders sleep in webs.

(The combat-hair will likely happen, no worries.)

Fixes #10616.
A really quick sketch:
```C++
static const std::array<body_part, 2> parts { { bp_head, ... } };
for( auto bp : parts ) {
    if( !wearing_something_on( bp ) ) {
        mod_dodge_bonus( +1 );
    }
}
```
You need to fill in the body parts in the first line and adjust the array size.