src/psl.c: Fix build on Visual Studio
src/psl.c: Fix build on Visual Studio

Visual Studio does not come with unistd.h, so only include it when we
have HAVE_UNISTD_H.  It also does not like including locale.h within
main() for some reason, so include that with the other system headers.

rockdaboot/libpsl,rockdaboot/libpsl,dago/libpsl,dago/libpsl,rockdaboot/libpsl,dago/libpsl