Bugfixes for Vacuum part in spin-spiral calculation. Still not 100% correct as conribution of last k+g that are only present for one spin are missing. Might not be relevant at all since these are expected to be tiny.

changed phas(n) in spgrot.f to cc. Use cc of pgfft in pwden.F90,
partially in fft(3,2)d(xc).f90, and of phas in cdnovlp.F90.
Introduce complex phase & derived variables in q_int_sl.f90, hsint.F90,
vacfun.f90, potdis.f90. Initializations in strgn.f90, real arguments
for dot_product() in pwint(_sl).f90 & phasy1.f90. Stop message in
rw_symfile.f changed to 'complex phases not fully tested'.

Based on v0.26 hybrid branch