rename strftime module

[Bruno Haible]

Hi,

The name of the strftime module is misleading:
  1) It suggests that it defines the function 'strftime', but in fact it
     defines 'nstrftime'. GNU findutils fell into this pit: their bootstrap.conf
     requests the gnulib 'strftime' module but the coded doesn't use nstrftime.
  2) When we need to apply workarounds to the strftime() function, it is
     awkward to not be able to use the module name 'strftime' for it.

I'd suggest to rename 'strftime' to 'strftime-gnu' or 'nstrftime', and mark
the 'strftime' module obsolete for two years, then after two years remove it.
So that all users of this module have enough time to notice the change.

Bruno