uudoc: add a feature to write mans and completions to the directory

[oech3]

We depend on GNU make to install them currently:

coreutils/GNUmakefile

Lines 218 to 226 in c24d032

	install-completions: build-uudoc
	$(INSTALL) -d $(DESTDIR)$(DATAROOTDIR)/zsh/site-functions
	$(INSTALL) -d $(DESTDIR)$(DATAROOTDIR)/bash-completion/completions
	$(INSTALL) -d $(DESTDIR)$(DATAROOTDIR)/fish/vendor_completions.d
	$(foreach prog, $(INSTALLEES), \
	$(BUILDDIR_UUDOC)/uudoc completion $(prog) zsh > $(DESTDIR)$(DATAROOTDIR)/zsh/site-functions/_$(PROG_PREFIX)$(prog) $(newline) \
	$(BUILDDIR_UUDOC)/uudoc completion $(prog) bash > $(DESTDIR)$(DATAROOTDIR)/bash-completion/completions/$(PROG_PREFIX)$(prog).bash $(newline) \
	$(BUILDDIR_UUDOC)/uudoc completion $(prog) fish > $(DESTDIR)$(DATAROOTDIR)/fish/vendor_completions.d/$(PROG_PREFIX)$(prog).fish $(newline) \
	)

This is one of a barriter to install coreutils on the system missing GNU make.