Batch of small 64-bit/long double/large file support tweaks:

Batch of small 64-bit/long double/large file support tweaks:
- scan for LDBL_DIG
- from DBL_DIG and LDBL_DIG select NV_DIG
- introduce IVSIZE, UVSIZE, NVSIZE
- introduce IV_DIG
- remove stdio64
- AIX uses `oslevel` when others use `uname -r`
- already AIX 4.2 goes 64-bit
- in HP-UX require the 64-bit libc, just the directory isn't enough
- group ids are not NVs
- #undef USE_LONG_DOUBLE if long double is no better than double
- introduce NV_WITHIN_*() and IV_FITS_IN_IV
- mention large file support in perldelta
- introduce quad TOPpin' and POPpin'
- the svcat... buffer was tiny for printing quads in %b
- fix the multiplication test in 64bit.t
- try to make VMS to comply with all this removal and "introducal"
  of symbols

p4raw-id: //depot/cfgperl@3995