regex: remove unnecessary type punning

regex: remove unnecessary type punning

Problem reported by Vladimir Serbinenko in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-04/msg00006.html>.
* lib/regex.h (struct re_pattern_buffer): Change the type of
__REPB_PREFIX(buffer) from unsigned char * to struct re_dfa_t *.
Fix comment to match code.
* lib/regcomp.c (re_compile_fastmap, re_compile_fastmap_iter, regfree)
(re_compile_internal, free_workarea_compile, analyze, lower_subexp)
(parse, parse_reg_exp, parse_branch, parse_expression, parse_sub_exp):
* lib/regexec.c (regexec, re_search_stub, re_search_internal)
(set_regs):
Omit no-longer-necessary casts.