fix for the bug under CygWin 20.1

[Igor Schein <igor@txc.com>]

Hi,

in order to have factorization work properly under CygWin,
you need to replace "r" with "rb" ( in 5 places ) and "w" with "wb"
( in 12 places ) in src/modules/mpqs.c for all pari_fopen() calls.
It probably should be #ifdef'ed in the next PARI release.

It has to do with the difference between UNIX and DOS text format,
which effect fseek() to the point where it goes berserk.

Igor