revert most of the CVE patch and “do it right”

directly after creating the new group home directory, as root,
there is no race that can appear due to *users* creating stuff
inside, so do not account for it; also make this code legible
SudoEffectiveUser needs unix_name, not uid, of the target

unbreaks group homedir creation for the n-th time
merge from Evolvis: some more variables and compat functions
merge from Evolvis: group home permission changes

• fallback if /usr/share/gforge/lib/private_default_page.php does
  not exist
• change index file and incoming directory to be group-writable
  (with sgid bit set) by default, to be actually useful
emit a newline after the warning