main: create new file when writing pidfile

When writing the pidfile, open the file with the O_CREAT|O_EXCL flags
to avoid following a symlink and writing the PID to an unexpected file,
when chronyd still has the root privileges.

The Linux open(2) man page warns about O_EXCL not working as expected on
NFS versions before 3 and Linux versions before 2.6. Saving pidfiles on
a distributed filesystem like NFS is not generally expected, but if
there is a reason to do that, these old kernel and NFS versions are not
considered to be supported for saving files by chronyd.

This is a minimal backport specific to this issue of the following
commits:
- commit 2fc8edacb8 ("use PATH_MAX")
- commit f4c6a00b2a ("logging: call exit() in LOG_Message()")
- commit 7a4c396bba ("util: add functions for common file operations")
- commit e18903a6b5 ("switch to new util file functions")

Reported-by: Matthias Gerstner <mgerstner@suse.de>

logging.c

@@ -171,6 +171,7 @@ void LOG_Message(LOG_Severity severity,
         system_log = 0;
         log_message(1, severity, buf);
       }
+      exit(1);
       break;
     default:
       assert(0);