print/hplip: Apply shebangfix to hpps and pstotiff filters

This was causing some print jobs to fail with this error in
/var/log/cups/error_log:
[Job XX] env: python: No such file or directory
[Job XX] PID XXXXX (/usr/local/libexec/cups/filter/hpps) stopped with status 127 (File too large)

Tidy up a few other items in the port Makefile whilst here.

MFH: 2017Q1