NM_ASCII_SPACES contains the ASCII characters according to
g_ascii_isspace().
Add NM_ASCII_WHITESPACES which differs from NM_ASCII_SPACES by not
including "\f". In some cases, that character shall be excluded.
For example, this is what systemd uses as "WHITESPACE" define at
various places.
Also, reorder the spaces string so that plain space comes first. It is
expected that ' ' is much more frequently than newlines or tabs. While
the order here shouldn't matter, it seems preferably to order frequent
characters in front.
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