ps(1): Constify the array of valid keywords

[olce]

Summary

Building on the previous commit that moved the 'width' field from VAR to
VARENT, it is now possible to constify the var[] array containing all
the recognized keywords.

As a side-effect, in parsefmt(), forget about allocating a new VAR for
the selected keyword to be pointed by the corresponding column (VARENT),
and instead just keep a pointer to that structure in var[].