tex-texdoc{,-doc}: update to 2019

This is Texdoc update aiming to be contained in TeX Live 2019:

- New option -c: changing configure temporally
- New config item `texlive_tlpdb` to specify a path for texlive.tlpdb
- Fixed an issue that texdoc fails to open gzipped file in GNOME/XFCE
- Added prefixes to script files not to contaminate kpse search space
- Many other small improvements and bug fixes

(markd)