tree-wide: increase granularity of percent specifications all over the place to permille

tree-wide: increase granularity of percent specifications all over the place to permille

We so far had various placed we'd parse percentages with
parse_percent(). Let's make them use parse_permille() instead, which is
downward compatible (as it also parses percent values), and increases
the granularity a bit. Given that on the wire we usually normalize
relative specifications to something like UINT32_MAX anyway changing
from base-100 to base-1000 calculations can be done easily without
breaking compat.

This commit doesn't document this change in the man pages. While
allowing more precise specifcations permille is not as commonly
understood as perent I guess, hence let's keep this out of the docs for
now.