Whitespace fixes

Mostly trailing spaces or whitespace in empty lines.
Use getifaddrs to get the interface-id when not specified

Reimplementing this section also fixes two potential bugs:

1. Only link-local addresses should be used for automatic interface-id
   selection, as it should always be assigned by the kernel and is always based
   on the EUI-64 unless an administrator is doing strange things (while other
   addresses might be chosen according to the privacy extensions or by an
   administrator)
2. When no address is available (because of a race condition or because an
   administrator removed it), RA packets should just be ignored; otherwise
   an address with a zero interface-id might be assigned
Fix race conditions in signal handling

Avoid several race conditions by using distinct variables for the different
signals. In particular different signals received in quick succession don't
overwrite each other any more, and odhcp6c_signal_process doesn't return true
anymore when a new SIGIO is received while another is still being processed.
Merge pull request #11 from NeoRaider/fixes

A few fixes for race conditions and similar problems