1) routines fwfft and invfft moved out of file fft_interfaces.f90 into new
   file fft_fwinv.f90. Prevents massive recompilation if something changes
   in the FFT routines.
2) machine-dependent fft_scalar.*.f90 are now modules with different names,
   conditionally included into fft_scalar with a USE, no longer an #include.
   Avoids trouble with dependencies, allow simplification of makedeps.sh.
All changes should be harmless, but I have tested only FFTW, FFTW3, DFTI.
Please let me know if there is any problem



git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@13185 c92efa57-630b-4861-b058-cf58834340f0