add TODO regarding Y2038 (main)
do not just see if it's there, make sure it is usable. Currently, alpine sys/tree.h generates code that doesn't work with both gcc and clang (for different reasons!) so make sure we can actually use it, or fall back on the bundled version.
Now `ls pref<TAB>' will complete all the paths starting with `pref' in the current directory; before it was only working for sub-directories.
no functional changes
in d444fd2 I forgot to rename HAVE_ERR to HAVE_ERRC too.
while here also fix the prototypes for warn and warnx