oping: Temporarily drop privileges if supported by the system.

oping: Temporarily drop privileges if supported by the system.

This tries to solve the read-from-any-file-issue more elegantly by
dropping privileges first thing after start-up and regaining privileges
just to open the sockets.

This has the advantage that the "-f" option is available for all users
again and files are opened according to *their* permissions.

Systems not supporting the _POSIX_SAVED_IDS feature still behave as
before, i. e. if real and effective user IDs don't match, only "-" may
be specified as input file.