Update for recent PHP shell changes

1. Ditch the PATH variable since it adds an extra layer of complexity to the additionally existing .local/bin/php approach
2. Introduce a ~/.local/bin/.lock_homephp file through which the user can force the use of the PHP binary in ~/.local/bin instead of /etc/alternatives/php in jails. Only Debian/Ubuntu systems should be less or more affected by that.
3. Update the server_php cli binary validators to make them more resilient to wrong or incomplete values.