build: avoid some compiler warnings

build: avoid some compiler warnings

* lib/selinux-at.h: Use dir_fd, not dirfd, to avoid shadowing.
* lib/exclude.c (fnmatch_pattern_has_wildcards): Use correct
type.
(new_exclude_segment, excluded_file_pattern_p)
(excluded_file_name_p): Reduce scope.
* lib/vasnprintf.c (decimal_point_char): Avoid warning on
old-style declaration.

Signed-off-by: Eric Blake <ebb9@byu.net>