#5870 Define Scheme markups using define-public

[Han-Wen Nienhuys]

Previously, define-markup-command[-list] would call module-define! on
(current-module). This is at odds with the GUILE 2.x precompilation,
as the side effect of module-define! is not serialized into the
compiled file.

Trim the is-list argument from define-markup-command-internal.

The define-markup-command-internal (which uses module-define) is
retained, because the parser uses it for defining markups.

Remove the undocumented and untested functionality to do

(define-markup-command a b)
(define-markup-command a (markup-lambda ..))

Because define-markup-command now reduces to a (define .. ) call, it
must happen at toplevel. This is a minor incompatible change.

Makes the function make-markup public

https://codereview.appspot.com/577720043

[Anonymous]

Passes make, make check and a full make doc.

The only reg test diffs are cell counts - but I never know if these are bad or good in the context of whatever a patch is.

So here they are anyway (attached)

[Han-Wen Nienhuys]

retain existing

https://codereview.appspot.com/577720043

[Anonymous]

Fails a make doc

Parsing...
Renaming input to: `accordion-register-symbols.ly'<unnamed port>: In expression (make-markup discant-markup "make-discant-markup" ...):
<unnamed port>: Unbound variable: make-markup
fatal error: Children (0) exited with errors.