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

while here also rearrange a bit the compl_state_reset/compl_add_entry dance and do the allocations only in the latter.