The print-output knows three modes: NORMAL, PRETTY, TERSE.
Instead of using local variables "pretty" and "terse",
check for the output mode directly.

Note how we have tree modes, so mapping them to two boolean
variables is confusing. Especially at one place where we did:
  pretty = (nmc_config->print_output != NMC_PRINT_TERSE);
while at other places:
  pretty = (nmc_config->print_output == NMC_PRINT_PRETTY);