Check for dirent.h and pwd.h.

Check for dirent.h and pwd.h.

* configure.in acconfig.h: Check for dirent.h and pwd.h.

* gtk/gtkfilesel.c: Use g_get_current_dir(). Merge in Win32
version: Use G_DIR_SEPARATOR, g_path_is_absolute, no tilde
expansion (based on HAVE_PWD_H), allow for drive letters UNC
names not handled yet. Also, code from Craig Setera's port to
Win32 that uses cygwin.dll