determine_executable_name doesn't do the job for FreeBSD

[xrme]

The function determine_executable_name() in the lisp kernel is supposed to return the full path to the running lisp kernel executable.

This function uses various system-specific ways to do this, depending on what the platform the lisp kernel is being compiled for. Unfortunately, the bit of code for FreeBSD doesn't do the job. It essentially just calls realpath(3) on the value of argv[0].

To see why this is inadequate:

$ ./fx86cl64
? (save-application "magic" :prepend-kernel t)
$ cp magic ~/bin    # or some other place in your path
$ cd /
$ magic
Couldn't load lisp heap image from magic.image: No such file or directory

This problem was discovered in dimitri/pgloader#551

[xrme]

Also merged to the 1.11 branch in fa63981