sort custom options into multiple custom groups and rearrange defintions

The `magit' custom group has gotten to full, split into several
subgroups.  Also rearrange the definitions of custom options in
`magit.el' into sections that more or less match the groups.

In other libraries add a new "Options" section unless that already
existed.  Move misplaced option definitions into that section.  Also
split the remainder of these libraries into sections where necessary,
to avoid adding a "(rest)" section (which indicates that it still has
to be split into multiple sections).
magit-blame-map: don't define as a constant
magit-blame-map: don't set menu string
magit-blame-sha1: cosmetics
add gitattributes-mode to magit-modes custom group
rename option magit-{ => process}-quote-curly-braces
magit-status: call magit-save-some-buffers

It used to do that but this got lost somewhere along the way.
magit-save-some-buffers: adjust doc-string

Update the option's doc-string; `magit-status' isn't the
only command that calls function `magit-save-some-buffers'.
rename function magit-{git => process}-quote-arguments
magit-process-quote-arguments: move definition
magit-{call, start}-git: call magit-process-quote-arguments

The processors of these functions used to do that
but this got lost somewhere along the way.
Merge branch 'pr/options'