Document possible color scheme resetting on :colo

Not obvious.

Thanks to y2kbugger.
Request full redraw in highlight_cmd()

Request full update instead of redraw to force recalculation of
mixed colors like cursor line, which otherwise are not updated.
Fix check for wrong color number in :highlight

Thanks to y2kbugger.
Fix flagging about file sourcing errors

Was accidentally removed?
Fix :highlight command recovery

Thanks to y2kbugger.
Move ui.c:load_color_scheme() to color_scheme.c

Did want to do this before, but since color_scheme.c already contains
check_directory_for_color_scheme(), it's OK.
Rename load_{,primary_}color_scheme()

This way it reflects its purpose better.
Rename "default color scheme" to "primary ..."

In documentation, to avoid confusion.
Extract color_scheme:restore_primary_color_scheme

Restore previous state of primary color scheme.
Better version of commands.c:get_color()

In multiple ways.
Break commands.c:highlight_cmd into smaller pieces
Change "colorscheme" to "color scheme" in docs
Merge branch 'several-cs-related-fixes'

Fix check for wrong color number in :highlight.

Fix :highlight command recovery.

Thanks to y2kbugger.