Improved fix 'typeset -xu', 'typeset -xl' (rhbz#1188377)

[hyenias]

'typeset -xu' and 'typeset -xl' would export the variable but fail
to change case in the value as the check between old and new
attributes did not provide the necesssary insight for lower or
upper case transcoding due to the lower or upper case attribute
being set within typeset.c prior to calling name.c nv_newattr
function.

Previous rhbz#1188377 patch added a conditional check for size==-1
which in effect caused the nv_newattr export code block return
optimization to never be executed as one cannot set any attributes
using the readonly builtin. By altering the size==-1 check to !trans
the export only optimization can run.

Also, the rhbz#1188377 patch altered new_attr function by setting
the new size to oldsize if run by the readonly builtin. The result
of setting size==oldsize allowed the succeeding if statement to
run more frequently and if size was a non-zero value resulted in
nv_setsize resetting the value to what it already was. Investigation
yielded that size was always 0 coming from the readonly builtin.

src/cmd/ksh93/bltins/typeset.c:

• Remove the setting of tdata.argnum to -1 as it is not needed due to
  existing name.c nv_newattr() logic.

src/cmd/ksh93/sh/name.c: nv_newattr():

• Corrected the export only check optimization by using !trans instead
  of using size==-1.
• Removed previous condition check to set size=oldsize if coming from
  the readonly builtin. nv_newattr already had existing logic to
  prevent changing the size via nv_setsize as size is always 0 when
  coming from readonly builtin.

[McDutchie]

Thanks for the improvement. Merging. Sorry for the delay.