update-xfonts-traditional: process fonts in parallel

update-xfonts-traditional: process fonts in parallel

Exploit multicore systems to speed up the runtime when processing
needs to be done.

Some timing results on a dual-core 1.50GHz Atom N550 laptop:

-j1
  real    0m43.035s
  user    0m43.319s
  sys     0m37.830s

-j2
  real    0m23.113s
  user    0m40.643s
  sys     0m30.486s

-j3
  real    0m21.910s
  user    0m43.707s
  sys     0m30.674s

-j4
  real    0m22.038s
  user    0m44.695s
  sys     0m31.058s

This system has hyperthreading so looks like 4 cores.  Based on this
I have set the number of processing threads to the number of cpus.