Make the behavior when we cannot write to a log or audit file

configurable.  File log failures are ignored by default for consistency
with syslog.  Audit errors are ignored by default to allow the admin
to fix the issue.  I/O log file errors are still fatal by default
since if I/O logging is activated it is usually to have an audit trail.
Bug #751

src/sudo.h

@@ -130,6 +130,7 @@ struct user_details {
 #define CD_SUDOEDIT_FOLLOW	0x10000
 #define CD_SUDOEDIT_CHECKDIR	0x20000
 #define CD_SET_GROUPS		0x40000
+#define CD_IGNORE_IOLOG_ERRS	0x80000
 
 struct preserved_fd {
     TAILQ_ENTRY(preserved_fd) entries;