drawbar: No need to reduce height of separator lines

Avoids rounding issues and stuff.
Use "nmaster" only if there's more than 3 clients

I find "nmaster = 2" very comfortable -- but only if there's enough
windows on screen. For two windows, it's awkward. For three windows,
it's unbalanced because the very first window sits alone in the slave
column and it is larger than the second window. For four or more
windows, "nmaster = 2" is actually useful.

This is an experimental change. I'll have to try this for a little
while. Eventually, I might turn it into a proper configuration option
(config.h).
Generalize previous commit: Make nmaster dynamic

- nmaster must be >= 1.
- Try to have as many clients in the master column as there are clients
  in the slave column. I *think* this is what I want. Monocle mode can
  zoom in on one particular client.
- Limit clients in the master column to 4.
- m->nmaster can override actual_nmaster if it's larger. This allows for
  some fine tuning in special situations.

This is still an experiment.
Modified dynamic nmaster: nmaster can override it

First, make nmaster_dynamic_max a proper setting.

Second, I think it's more likely that I want to set nmaster manually
(instead of just being able to set it to something *larger* than
nmaster_dynamic_max).
Dynamic nmaster is not an experiment anymore