* eval.c: define scm_unbound_variable_key ('unbound-variable).

	* eval.c: define scm_unbound_variable_key ('unbound-variable).
	scm_lookupcar1: throw an error with key 'unbound-variable instead
	of 'misc-error when an unbound variable is encountered.

	* filesys.c (scm_mkdir, scm_rmdir, scm_getcwd, scm_select,
	scm_symlink, scm_readlink, scm_lstat),
	posix.c (scm_setpgid, scm_setsid, scm_ctermid, scm_tcgetpgrp,
	scm_tcsetpgrp, scm_uname, scm_setlocale, scm_mknod, scm_nice,
	scm_sync),
	simpos.c (scm_system),
	stime.c (scm_times, scm_strptime):
	move the HAVE_XXX feature tests out of the procedure bodies.
	don't use SCM_SYSMISSING.
	scm_validate.h (SCM_SYSMISSING): removed.
	error.h, error.c (scm_sysmissing): comment that this is deprecated.
	see ChangeLog entry for 1999-12-28.