refactor: zmacs is an existing editor project; rename ours to zemacs.

Substitute all occurrences of zmacs, Zmacs and ZMACS with zemacs,
Zemacs and ZEMACS respectively.
* bin/zmacs, lib/zmacs, lib/zmacs/mkdotzmacs.lua,
lib/zmacs/zmacs.mk: Move from here...
* bin/zemacs, lib/zemacs, lib/zemacs/mkdotzemacs.lua,
lib/zemacs/zemacs.mk: ...to here.
Adjust all clients.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specs: be sure to set batch_mode flag before first use.

* lib/zemacs/specs/spec_helper.lua (edit): Be sure to set the
batch_mode flag before calling mkmacro/mklisp functions that
require it.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specs: add specifications for point, point-max and point-min.

* lib/zeemacs/specs/zlisp_buffer_spec.yaml: Specify behaviour of
`point`, `point-max` and `point-min` lisp functions.
* lib/zemacs/specs/specs.mk (specl_SPECS): Add
lib/zeemacs/specs/zlisp_buffer_spec.yaml.
* lib/zeemacs/default-bindings-el.lua (global-set-key): Add
missing \M-gc binding for goto-char, required by new specs.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>