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
daemonp: using freopen doesn't guarantee proper file descriptors

Use close and open explicitly.  freopen isn't required to reuse the same
file descriptors according to something I read online.
daemonp: call setsid in the radvd child process

This makes the radvd child process a new session and group leader.
daemonp: set radvd umask to zero

daemons typically set their umask to zero and radvd is a typical daemon.
privsep-linux.c: umask(0), setsid(), chdir("/")

privsep_init should ensure its umake is zero, it's a session/group leader
and to change dir to '/' to act like a proper daemon.  I'd like the
privsep process to be in the same session/group as the radvd child process,
but I always was to fork off privsep before the config file is read.
Forking after reading the config file uses extra memory in privsep for no
good reason.
privsep-linux.c: stdin, stdout, stderr -> /dev/null

This was already happening, but in a different way.  This normalizes the
code to match what's in daemonp.
privsep after daemonp
radvd.c: close pidfile after validated
radvd.c: check return values of write and chdir
daemonp: don't exit in parent

This allows signalling back up the call stack to allow the parent
to free memory before exiting.  This ensures that memory leaks
reported by valgrind is really leaks.
Merge branch 'privsep_daemonp'
update configure.ac and CHANGES for v2.7