This commit factorizes some code in lib/conf.c from the two hosted
startup files, but the aim is passing "-f <filename>" on the command
line, to test with unconventional setups without touching the "real"
filesystem.

Now ppsi looks for the configuration file is from threee places, in
this order:
    - command line, if any
    - architecture-specific, if any (e.g. "/wr/etc/ppsi.conf")
    - compile-time default ("/etc/ppsi.conf")
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>