Fix memory leaks in scanners generated by at least Flex 2.5.9 and

Fix memory leaks in scanners generated by at least Flex 2.5.9 and
later.  Reported by Paul Eggert in
<http://lists.gnu.org/archive/html/bison-patches/2006-11/msg00014.html>.
* src/flex-scanner.h (yylex_destroy): Define for Flex before 2.5.9.
* src/scan-code.l (translate_action): Don't bother invoking
yy_delete_buffer (YY_CURRENT_BUFFER) before creating the first buffer.
(code_scanner_free): Instead of invoking
yy_delete_buffer (YY_CURRENT_BUFFER) directly, invoke yylex_destroy,
which frees more.
* src/scan-gram.l (gram_scanner_free): Likewise.
* src/scan-skel.l (scan_skel): Likewise.