* net/tramp.el (tramp-perl-file-truename): New defconst. Perl

* net/tramp.el (tramp-perl-file-truename): New defconst.  Perl
code contributed by yary <not.com@gmail.com> (tiny change).
(tramp-handle-file-truename, tramp-get-remote-perl): Use it.
Check also for "perl-file-spec" and "perl-cwd-realpath"
properties.
(tramp-handle-write-region): In case of APPEND, reuse the tmpfile
name.

* net/tramp.el (tramp-perl-file-name-all-completions): New
defconst.
(tramp-get-remote-readlink): New defun.
(tramp-handle-file-truename): Use it.
(tramp-handle-file-exists-p): Check file-attributes cache, assume
file exists if cache value present.
(tramp-check-cached-permissions) New defun.
(tramp-handle-file-readable-p): Use it.
(tramp-handle-file-writable-p): Likewise.
(tramp-handle-file-executable-p): Likewise.
(tramp-handle-file-name-all-completions): Try using Perl to get
partial completions.  When perl not available, combine `cd' and
`ls' into single remote operation and use shell expansion to get
partial remote directory contents.  Set `file-exists-p' cache for
directory and any files returned by ls.  Change cache handling to
support partial directory contents.  Use error message emitted by
remote `cd' or Perl code for local tramp-error.
(tramp-do-copy-or-rename-file-directly): Avoid separate
tramp-send-command-and-check call.
(tramp-handle-process-file): Merge three remote ops into one.  Do
not flush all caches when `process-file-side-effects' is set.
(tramp-handle-write-region): Avoid tramp-set-file-uid-gid if
file-attributes shows uid/gid to be set already.