Older versions of GNU gcc do not allow 'NULL' as sentinel.

They want (char *)NULL as sentinel.
An example is OpenBSD (confirmed on 4.3) that ships with gcc 3.3.4

This commit introduces a contstant SENTINEL which is declared as:
#define SENTINEL ((char *)NULL)

All places I could test compile on my openbsd system are converted.
Update CODING-GUIDELINES to tell about this constant.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@124127 65c4cc65-6c06-0410-ace0-fbb531ad65f3