Expand host names in Tramp's ad-hoc multi-hop file names

Expand host names in Tramp's ad-hoc multi-hop file names

* doc/misc/tramp.texi (Quick Start Guide): Improve wording.
(Change file name syntax): Say, that `tramp-file-name-regexp' is
not constant.
(Ad-hoc multi-hops): Explain host name expansion.

* etc/NEWS: Mention that host names in Tramp ad-hoc multi-hop file
names must match the previous hop for methods like "su" or "sudo".
Fix typos.

* lisp/net/tramp.el (tramp-find-method, tramp-find-user):
Adapt docstring.
(tramp-find-host): Mark default value.
(tramp-dissect-file-name): Expand host name for hops.
(tramp-dissect-hop-name, tramp-make-tramp-hop-name): New defuns.
(tramp-clear-passwd): Simplify.

* test/lisp/net/tramp-tests.el (tramp-test02-file-name-dissect)
(tramp-test02-file-name-dissect-simplified)
(tramp-test02-file-name-dissect-separate)
(tramp-test26-file-name-completion): Extend tests.