Fix errors reported by cus-test.el

Fix errors reported by cus-test.el

* ox.el (org-export-async-init-file): Use :set to set the
default value.

* ox-texinfo.el (org-texinfo-filename): Fix default value.
(org-texinfo-format-headline-function): Use 'ignore as the
default value.
(org-texinfo-format-drawer-function): Use a function as the
default value.  Update docstring.
(org-texinfo-drawer): Always use
`org-texinfo-format-drawer-function' as it is now a function
by default.
(org-texinfo-headline): Compare
`org-texinfo-format-headline-function' against 'ignore.
(org-texinfo-inlinetask): Compare
`org-texinfo-format-inlinetask-function' against 'ignore.

* ox-odt.el (org-odt-format-drawer-function): Use a function
as the default value.  Update docstring.
(org-odt-format-headline-function)
(org-odt-format-inlinetask-function): Fix default value.
(org-odt-with-latex): Use :set to set the default value.
(org-odt-drawer): Always use `org-odt-format-drawer-function'
as it is now a function by default.
(org-odt-format-headline--wrap): Compare
`org-odt-format-headline-function' against 'ignore.

* ox-latex.el (org-latex-format-drawer-function): Use a
function as the default value.  Update docstring.
(org-latex-format-inlinetask-function): Fix default value.
(org-latex-drawer): Always use
`org-latex-format-drawer-function' as it is now a function by
default.
(org-latex-inlinetask): Compare
`org-latex-format-inlinetask-function' against 'ignore.

* ox-html.el (org-html-format-drawer-function): Use a
function as the default value.  Update docstring.
(org-html-format-headline-function)
(org-html-format-inlinetask-function): Fix default value.
(org-html-with-latex): Use :set to set the default value.
(org-html--format-toc-headline)
(org-html-format-headline--wrap): Compare
`org-html-format-headline-function' against 'ignore.
(org-html-inlinetask): Compare
`org-html-format-inlinetask-function' against 'ignore.

* ox-ascii.el (org-ascii-format-drawer-function): Use a
function as the default value.  Update docstring.
(org-ascii-drawer): Always use
`org-ascii-format-drawer-function' as it is now a function by
default.
(org-ascii-format-inlinetask-default): New function.
(org-ascii-format-inlinetask-function): Use
`org-ascii-format-inlinetask-default' as the default.

* org.el (org-mouse-1-follows-link): Use :set to set the
default value.  Update custom type.
(org-log-note-headings): Fix order or list items in the custom
type.
(orgstruct-heading-prefix-regexp): Use an empty string as the
default value.  Use 'regexp as the custom type.
(orgstruct-make-binding): Tiny docstring enhancement.  Assume
`orgstruct-heading-prefix-regexp' is a string.

* org-agenda.el (org-agenda-search-view-max-outline-level):
Set default value to 0.  Update docstring.
(org-agenda-deadline-leaders): Fix custom type.
(org-search-view): Assume
`org-agenda-search-view-max-outline-level' is a number.

* ob-ruby.el (org-babel-ruby-nil-to): Fix custom type.

* ob-python.el (org-babel-python-mode): Use :set to set the
default value.
(org-babel-python-None-to): Fix custom type.

* ob-plantuml.el (org-plantuml-jar-path): Fix default value.
(org-babel-execute:plantuml): Assume `org-plantuml-jar-path'
is a string.

* ob-latex.el (org-babel-latex-htlatex): Fix default value.
(org-babel-latex-htlatex-packages): Fix custom type.
(org-babel-execute:latex): Assume `org-babel-latex-htlatex' is
a string.

Thanks to Glenn Morris for reporting this.