GNU tools: improvements to fetching process

- Do not run the fetch.sh script every time
- Do not run the configure script with MKUPDATE=yes (build.sh -u)
- The fetched GNU source are hidden from `git status`
- With CLEANFETCHED=yes, `nbmake -C tools cleandir` will remove the
fetched GNU source and return the MINIX tree to its pristine state.
- Avoid (harmless) message when building in parallel, about find
running against an inexistant path

Notice that the GNU tools are "installed" on every run, even with
MKUPDATE=yes; this is a feature of the NetBSD build system, because
in case of changes in dependencies it is impossible to know them.
See commit 1.27 2004-06-12 of tools/Makefile.gnuhost in NetBSD's src/

The downloaded tarballs are not removed with CLEANFETCHED:
they are supposed to be unmodified, and the exact name is not known
to the Makefile's, since it is engraved in each fetch.sh script (and
we do not want to have the same information in several places.)
mkdep: restore NetBSD behaviour

Since MINIX has a working cc, do not introduce spurious differences
Ramdisk: remove mtab, as now obsolete
Replace tell_config with NetBSD osrelease.sh
make: correct a typo of NetBSD source
Mined: allow Ins to mark
import tput from NetBSD

Posix tool to allow direct use of the terminfo database.
The clear script is adjusted accordingly.
The (MINIX-specific) clr equivalent is now gone.
Import infocmp from NetBSD

Convert the content of the terminfo database into human-readable
form, the same as used by the tic(1) command already present.