radvd.c: add check_pid_file function
radvd.c: add daemonp function
radvd.c: factor out open_pid_file
radvd.c: use new daemonp and check_pid_file funcs
Merge branch 'pidfile2'
normalize some logging

Make logging short messages rather than complete sentences.  Complete
sentences are nice, but people don't want to read radvd output like a
book, so keep messages short, simple and make them read like newspaper
headlines and not like prose English.
release v2.6