ps(1): Have parsefmt() take the list of columns to update

[olce]

Summary

This is in preparation for changing the behavior of the '-O' option.

While here, reformat the definition of 'struct varent', fix formatting
of that of 'struct var' and expand slightly their herald comments.

No functional change intended.

More reformatting/commenting in 'ps.h'.

[kib]

bin/ps/keyword.c

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