Rework time-* functions in Tramp

Rework time-* functions in Tramp

* doc/misc/emacs-mime.texi (time-date): Add time-equal-p.

* lisp/net/tramp-compat.el (tramp-compat-time-equal-p): New defsubst.

* lisp/net/tramp.el (tramp-file-name-handler): Remove `debug'
error handler.
(tramp-half-a-year): Remove.
(tramp-time-dont-know, tramp-time-doesnt-exist): New defconst.
(tramp-time-diff): Remove compat code.
(tramp-handle-set-visited-file-modtime)
(tramp-handle-verify-visited-file-modtime):
* lisp/net/tramp-adb.el (tramp-do-parse-file-attributes-with-ls)
(tramp-adb-handle-set-file-times):
* lisp/net/tramp-sh.el (tramp-do-file-attributes-with-ls)
(tramp-sh-handle-set-visited-file-modtime)
(tramp-sh-handle-verify-visited-file-modtime)
(tramp-sh-handle-set-file-times)
(tramp-sh-handle-file-newer-than-file-p):
Use `tramp-time-dont-know', `tramp-time-doesnt-exist' and
`tramp-compat-time-equal-p'.
(tramp-sh-handle-verify-visited-file-modtime): Simplify check.

* lisp/net/tramp-smb.el (tramp-smb-handle-file-attributes)
(tramp-smb-read-file-entry): Use `tramp-time-dont-know'.
(tramp-smb-handle-insert-directory): Adapt half-a-year check.

* src/editfns.c (Ftime_equal_p): Adapt docstring.

* test/lisp/net/tramp-tests.el
(tramp-test19-directory-files-and-attributes)
(tramp-test22-file-times): Use `tramp-compat-time-equal-p' and
`tramp-time-dont-know'.
(tramp-test23-visited-file-modtime): Extend test.