cache: document negative ttls and add about ttl

We've long supported negative ttls, for infinite cache, except the
documentation incorrectly showed one of our defaults as being 5 and not
-1. As well, with a negative ttl, we were actually making the HTTP
expired header go backwards. This changes it to go ahead ten years
instead.

Further, we add an cache-about-ttl option to set a different ttl for
about pages, which are now increasingly being filtered through markdown
or just sent statically anyway.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
cache: id means static, even if head is specified too

Pages like /commit?h=wip&id=8a335ce618ba77fbf05148d6f8be17bd48ba4340
were being marked as dynamic, because of h=wip, when it should be
static, because of id=.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>