fileio: add READ_FULL_FILE_UNHEX flag

Similar to READ_FULL_FILE_UNBASE64, read data is decoded with
unhexmem().

src/basic/fileio.h

@@ -31,6 +31,7 @@ typedef enum {
 typedef enum {
         READ_FULL_FILE_SECURE   = 1 << 0,
         READ_FULL_FILE_UNBASE64 = 1 << 1,
+        READ_FULL_FILE_UNHEX    = 1 << 2,
 } ReadFullFileFlags;
 
 int write_string_stream_ts(FILE *f, const char *line, WriteStringFileFlags flags, struct timespec *ts);