Fix Z(6,3) segfaulting, and incorrect spelling of "finfield.c"

[fingolfin]

The crash occurred because upon calling Z(q,d), the kernel function FuncZ2 simply computes q^d, and only checks whether it is less or equal 65536, but does not verify that it is a prime power. This is not so bad, as it next calls FiniteField, which actually catches this -- and return 0. But then FuncZ2 blindly passes that value to NEW_FFE, which returned an invalid FFE object. The segfault then happens once GAP tries to do anything with that object, e.g. print it to the user...

The other commit replaces finifield.c by finfield.c inside GVarFilts[], GVarFuncs[] and InitKernel(). These strings are printed if you do e.g. Display(Z) in GAP, and having a correct filename helps with quickly locating the source code... :)

[codecov-io]

Current coverage is 49.46% (diff: 66.66%)

Merging #1029 into master will decrease coverage by 0.04%

@@             master      #1029   diff @@
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  Files           424        424          
  Lines        223266     223268     +2   
  Methods        3430       3430          
  Messages          0          0          
  Branches          0          0          
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- Hits         110545     110446    -99   
- Misses       112721     112822   +101   
  Partials          0          0          

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[olexandr-konovalov]

Both changes are useful and tests passed - I will just wait in case anyone else would like to comment.