pp_formline: handle growing better

pp_formline: handle growing better

We initially grow PL_formtarget by the largest amount a formline can
append (since formats are fixed width). The only thing that can exceed
that is @*; but also, if PL_formtarget gets upgraded to utf8, then some of
the extra buffer we allocated can get eaten up by the upgrade.

Codify this so we only grow when necessary, but always enough.

Prior to this commit, we were growing FF_ITEM/FF_LITERAL too much, and
FF_LINEGLOB too little (the latter being my mistake from a few commits
ago).

Also rename 'fudge' to 'linemax', to give a better idea what it's for.