Settings: preserve empty header

If the user removes all meters from the header on exit htop will write
no values for the settings column_meters_X and column_meter_modes_X.
The parser skips all configuration settings with no value, so on the
next start no header meter related key is processed and htop will add a
set of default meters to the header.

Write instead an invalid value of `!`, so the keys are parsed and htop
will not add the default meters back.

Closes: #1248

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