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>

util.h

@@ -176,6 +176,17 @@ extern int UTI_CreateDirAndParents(const char *path, mode_t mode, uid_t uid, gid
    permissions and its uid/gid must match the specified values. */
 extern int UTI_CheckDirPermissions(const char *path, mode_t perm, uid_t uid, gid_t gid);
 
+/* Open a file.  The full path of the file is constructed from the basedir
+   (may be NULL), '/' (if basedir is not NULL), name, and suffix (may be NULL).
+   Created files have specified permissions (umasked).  Returns NULL on error.
+   The following modes are supported (if the mode is an uppercase character,
+   errors are fatal):
+   r/R - open an existing file for reading
+   w/W - open a new file for writing (remove existing file)
+   a/A - open an existing file for appending (create if does not exist) */
+extern FILE *UTI_OpenFile(const char *basedir, const char *name, const char *suffix,
+                          char mode, mode_t perm);
+
 /* Set process user/group IDs and drop supplementary groups */
 extern void UTI_DropRoot(uid_t uid, gid_t gid);