Petty clean-up in serverd's server code.

I had to read it in order to make sense of what's going on with
call-collect; and reading can be made easier by making the code
cleaner !  Some long lines split, some comparisons to boolean reworked
to use booleans as booleans, some TODO in empty blocks, flipped a
conditional to put its simpler branch first, moved an assignment
within condition to a simple initializer for its variable.
Described a possibly better way of limiting how many threads we have.
Fixed a compiler warning, matching argument type to format specifier.