Improve make-process in Tramp

Improve make-process in Tramp

* doc/misc/tramp.texi (Remote processes): Remove INSIDE_EMACS
restriction.
(Frequently Asked Questions, External packages): Add indices.

* etc/NEWS: 'start-process-shell-command' and
'start-file-process-shell-command' do not support the old calling
conventions any longer.

* lisp/subr.el (start-process-shell-command)
(start-file-process-shell-command): Remove old calling conventions.

* lisp/net/tramp-compat.el (remote-file-error): Remove, it isn't
necessary.

* lisp/net/tramp.el (tramp-handle-make-process): Remove special shell
handling.  Support environment variables.

* test/lisp/net/tramp-tests.el
(tramp--test--deftest-direct-async-process): Skip for mock method.
(tramp--test-async-shell-command): Suppress `shell-command-sentinel'.
(tramp-test32-shell-command, tramp-test33-environment-variables):
Adapt tests.
(tramp-test32-shell-command-direct-async)
(tramp-test33-environment-variables-direct-async): New tests.