Improve handling of defcustoms in tramp

Improve handling of defcustoms in tramp

* doc/misc/tramp.texi: Protect all multi-line examples by
@group ... @end group.  Use proper `custom-set-variables'
examples.  Use consequently @option for user options.  Remove
superfluous @cindex and @vindex entries.
(Top): Remove reference to outdated mail archives.
(Configuration): Tune references.
(Default Method): Make `tramp-default-method' and
`tramp-default-method-alist' a @defopt.
(Default User): Make `tramp-default-user' and
`tramp-default-user-alist' a @defopt.
(Default Host): Make `tramp-default-host' and
`tramp-default-host-alist' a @defopt.

* lisp/net/tramp.el (tramp-mode, tramp-verbose)
(tramp-backup-directory-alist, tramp-auto-save-directory)
(tramp-encoding-shell, tramp-encoding-command-switch)
(tramp-encoding-command-interactive, tramp-default-method)
(tramp-default-method-alist, tramp-default-user)
(tramp-default-user-alist, tramp-default-host)
(tramp-default-host-alist, tramp-default-proxies-alist)
(tramp-save-ad-hoc-proxies)
(tramp-restricted-shell-hosts-alist)
(tramp-local-end-of-line, tramp-rsh-end-of-line)
(tramp-login-prompt-regexp, tramp-shell-prompt-pattern)
(tramp-password-prompt-regexp, tramp-wrong-passwd-regexp)
(tramp-yesno-prompt-regexp, tramp-yn-prompt-regexp)
(tramp-terminal-prompt-regexp)
(tramp-operation-not-permitted-regexp)
(tramp-copy-failed-regexp, tramp-process-alive-regexp)
(tramp-syntax, tramp-chunksize)
(tramp-process-connection-type, tramp-connection-timeout)
(tramp-connection-min-time-diff)
(tramp-completion-reread-directory-timeout):
* lisp/net/tramp-adb.el (tramp-adb-program)
(tramp-adb-connect-if-not-connected, tramp-adb-prompt):
* lisp/net/tramp-cache.el (tramp-connection-properties)
(tramp-persistency-file-name):
* lisp/net/tramp-gvfs.el (tramp-gvfs-methods)
(tramp-gvfs-zeroconf-domain)
(tramp-bluez-discover-devices-timeout):
* lisp/net/tramp-sh.el (tramp-inline-compress-start-size)
(tramp-copy-size-limit, tramp-terminal-type)
(tramp-histfile-override)
(tramp-use-ssh-controlmaster-options, tramp-remote-path)
(tramp-remote-process-environment, tramp-sh-extra-args):
* lisp/net/tramp-smb.el (tramp-smb-program, tramp-smb-acl-program)
(tramp-smb-conf, tramp-smb-winexe-program)
(tramp-smb-winexe-shell-command)
(tramp-smb-winexe-shell-command-switch): Add :require 'tramp.