chunk: Use dynamically allocated buffer in chunk_from_fd()

When acting on files, we can use fstat() to estimate the buffer size. On
non-file FDs, we dynamically increase an allocated buffer.

Additionally we slightly change the function signature to properly handle
zero-length files and add appropriate unit tests.

src/pki/commands/print.c

@@ -22,6 +22,7 @@
 #include <selectors/traffic_selector.h>
 
 #include <time.h>
+#include <errno.h>
 
 /**
  * Print public key information
@@ -510,7 +511,11 @@ static int print()
 	{
 		chunk_t chunk;
 
-		chunk = chunk_from_fd(0);
+		if (!chunk_from_fd(0, &chunk))
+		{
+			fprintf(stderr, "reading input failed: %s\n", strerror(errno));
+			return 1;
+		}
 		cred = lib->creds->create(lib->creds, type, subtype,
 								  BUILD_BLOB, chunk, BUILD_END);
 		free(chunk.ptr);