* When fixing up hash-bang lines, `::FS` returns a status integer, which was returned by the process
* also avoid expanding Make variables into TECO code - better pass it as a command-line argument
* should also fix OBS builds

The latter is apparently frowned upon.
This will hopefully get the v2.5.0 release into ports quicker.

* hash-bang lines in grosciteco.tes and tedoc.tes are fixed up
  at build time instead of install-time (via install-exec-hook).
* This will hopefully resolve problems with the FreeBSD port,
  since Poudriere installs the package as non-root but with BINMODE=555.
  The alternative would have been to set BINMODE=755 in the port which may not
  be acceptible by the official port committer.
  Anyway, I decided to go out of the way of any conflicts.
  See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?format=multiple&id=283601
* On the downside, this will break install-path overrides at install-time
  (e.g. `make install bindir=...`).

It causes build failures on OBS for several Linux distributions.
It generally works on Linux, so it's probably related to Xvfb.

Hopefully fixes the OBS build errors.

This is useful to launch from a terminal without "blocking" this terminal.
There are tools like nohup and daemonize (BSD) to do the same, but having it
builtin is shorter to write.