cmdline-opts/gen.pl: Open input files in CRLF mode

On Windows it's possible to have input files with CRLF line endings and
a perl that defaults to LF line endings (eg msysgit). Currently that
results in generator output of mixed line endings of CR, LF and CRLF.

This change fixes that issue in the most succinct way by opening the
files in :crlf text mode even when the perl being used does not default
to that mode. (On operating systems that don't have a separate text mode
it's essentially a no-op.) The output continues to be in the perl's
native line ending.