Change default errno value to 0 (no error) and add prototypes in lvm.h

Since we are using errno values, we should use '0' as a default value
which indicates a non-error, rather than defining some made-up default
value that is not defined in errno.  If we need to deviate from errno
values, this will most likely indicate a flaw in our design.

Add prototypes for lvm_errno and lvm_errmsg inside lvm.h and provide
a basic description of their function.  This fixes a couple compile
warnings.


Author: Dave Wysochanski <dwysocha@redhat.com>

lib/log/lvm-logging.h

@@ -16,14 +16,12 @@
 #ifndef _LVM_LOGGING_H
 #define _LVM_LOGGING_H
 
-#define EUNCLASSIFIED -1	/* Generic error code */
-
 void print_log(int level, const char *file, int line, int dm_errno,
 	       const char *format, ...)
     __attribute__ ((format(printf, 5, 6)));
 
 #define LOG_LINE(l, x...) \
-    print_log(l, __FILE__, __LINE__ , EUNCLASSIFIED, ## x)
+    print_log(l, __FILE__, __LINE__ , 0, ## x)
 
 #define LOG_LINE_WITH_ERRNO(l, e, x...) \
     print_log(l, __FILE__, __LINE__ , e, ## x)

liblvm/lvm.h

@@ -71,6 +71,30 @@ void lvm_destroy(lvm_t libh);
  */
 int lvm_reload_config(lvm_t libh);
 
+/**
+ * Return stored error no describing last LVM API error.
+ *
+ * Users of liblvm should use lvm_errno to determine success or failure
+ * of the last call, unless the API indicates another method of determining
+ * success or failure.
+ *
+ * \param   libh
+ *          Handle obtained from lvm_create.
+ *
+ * \return  An errno value describing the last LVM error.
+ */
+int lvm_errno(lvm_t libh);
+
+/**
+ * Return stored error message describing last LVM error.
+ *
+ * \param   libh
+ *          Handle obtained from lvm_create.
+ *
+ * \return  An error string describing the last LVM error.
+ */
+const char *lvm_errmsg(lvm_t libh);
+
 /**
  * Create a VG with default parameters.
  *