uip/folder.c: chdir(nmhdir) in main rather than in dodir, which

uip/folder.c: chdir(nmhdir) in main rather than in dodir, which
is called many times.  Drop the chdir in get_folder_info, which
served no purpose at all.  If struct stat has d_type (BSD), use
that to skip processing (strcmp, stat, and so on) of message
files, mostly undoing the slow-down from the last change.
Plus autoconf machinery to detect d_type field. Patch from Eric Gillespie.