Specter cannot reopen logs owned by root after receiving HUP signal

Owner of all new log files created is always root, even when --uid/--gid command line options are used. This means that after dropping root priviledges specter won't be able to reopen and write to them.

Two changes to specter are neede to fix that:
 * document the behavior of specter in relation to new and existing files, including usage of --uid/--gid and HUP signals
 * make specter create files owned by --uid/--gid provided on the command line, so reopening on HUP still works