Header and comments describing routines in incorrect shape in visibilitymap.c

Hi all,

I just bumped into a couple of things in visibilitymap.c:
- visibilitymap_clear clears all bits of a visibility map, its header
comment mentions the contrary
- The header of visibilitymap.c mentions correctly "a bit" when
referring to setting them, but when clearing, it should say that all
bits are cleared.
- visibilitymap_set can set multiple bits
- visibilitymap_pin can be called to set up more than 1 bit.

This can be summed by the patch attached.
Regards,
--
Michael