Recommend wrappers of PG_DETOAST_DATUM_PACKED()

When commit 3e23b68dac006e8deb0afa327e855258df8de064 introduced single-byte
varlena headers, its fmgr.h changes presented PG_GETARG_TEXT_PP() and
PG_GETARG_TEXT_P() as equals. Its postgres.h changes presented
PG_DETOAST_DATUM_PACKED() and VARDATA_ANY() as the exceptional case. Let's
firmly recommend PG_GETARG_TEXT_PP() over PG_GETARG_TEXT_P(); likewise for
other ...PP() macros. This shaves cycles and brings consistency of style.

If the attached policy-setting patch seems reasonable, I will follow it with a
mechanical patch adopting the recommended macros throughout the tree. (If any
code does want the alignment it gets from an obsolecent macro, I will add a
comment to that code.)

Thanks,
nm