Minor bugfixes, .pgen forward-compatibility

Bugfixes include:

• --update-sex no longer forgets to print an error message when file-open fails.
• --no-categorical no longer causes a segfault or assertion failure when the first nonheader line of a phenotype/covariate file has a category name.
• --pgen-info now writes to stdout instead of stderr.

This also makes the .pgen reader compatible with the header/footer extensions permitted by the May 2024 .pgen specification update, and backports the --set-invalid-haploid-missing and --neg9-pheno-really-missing flags.