nbd-server: remove return statement after call to serveloop()

serveloop() never returns, now it is also marked as such.

Signed-off-by: Tuomas Jorma Juhani Räsänen <tuomasjjrasanen@tjjr.fi>
nbd-server: fix parse_cfile() to not modify any global variables

This change allows parse_cfile() to be called without any side-effects
and passes the decision of what to do with parsed results to the
caller.

Signed-off-by: Tuomas Jorma Juhani Räsänen <tuomasjjrasanen@tjjr.fi>
nbd-server: remove global variables: runuser and rungroup

User and group names are needed only at the very beginning of the
process, there is no need to keep them hanging in the global scope.

Signed-off-by: Tuomas Jorma Juhani Räsänen <tuomasjjrasanen@tjjr.fi>
nbd-server: remove global variables: modern_listen and modernport

Address and port of the modern socket are read from the main
configuration file and used only when opening the socket. There is no
need to keep them hanging in the global scope.

Signed-off-by: Tuomas Jorma Juhani Räsänen <tuomasjjrasanen@tjjr.fi>
Remove unnecessary free() calls

Technically this is a leak, but it does no harm to leave them there:
- It may aid in debugging
- future features (try pronouncing that! ;-) may use these values for
  other purposes
- additionally, while the pointers are freed, they're not set to NULL,
  which makes them unsafe dirty pointers. That's actually worse than not
  being freed, in my opinion.