include: map VLC_EGENERIC to INT_MIN

VLC_EGENERIC has no equivalent in <errno.h>, so it should not alias any
<errno.h> value. This maps it to INT_MIN. It works because -INT_MIN equals
INT_MAX + 1, which is out of range of `int`. As a reminder codes in
<errno.h> are positive integers.

The convoluted definition is to avoid pulling <limits.h> into
<vlc_common.h> and from there all over the code base.

include/vlc_common.h

@@ -469,7 +469,7 @@ typedef union
 /** No error */
 #define VLC_SUCCESS        (-0)
 /** Unspecified error */
-#define VLC_EGENERIC       (-1)
+#define VLC_EGENERIC       (-2 * (1 << (sizeof (int) * 8 - 2))) /* INT_MIN */
 /** Not enough memory */
 #define VLC_ENOMEM         (-2)
 /** Timeout */