Re-factor service widths

* Specify service road widths as variables
* Make service roads on z16 more narrow (solves 3499 on trac)
* Specify service bridge widths in the same way as other service widths
* Remove default value line-cap=butt
* Do not render minor service-tunnels on z14/z15
Merge pull request #649 from math1985/service-widths

Re-factor service widths