[BUG] gtags-find-rtag doesn't work through GTAGSLIBPATH

[Gini]

- under emacs, gtags-find-rtag doesn't work through GTAGSLIBPATH
- to overcome this you must first gtags-find-tag and then at the point
of definition
execute again gtags-find-rtag ... which is not ok

The following environment was setup:

1. the directory was created $HOME/work/linux-2.6.26 with the linux
kernel sources
2. the file was created $HOME/work/modules/hello_word.c
3. global installed via apt-get install global
4. in directory $HOME/work/linux-2.6.26 the gtags command was executed and
the 4 G* files ware created
5. in directory $HOME/work/modules the gtags command was exectuted and
the 4 G* files ware created
6. in the shell export GTAGSLIBPATH=$HOME/work/linux-2.6.26 was executed
7. in the same shell we do:
$ cd $HOME/work/modules
$ emacs hellow_word.c
Under emacs editor do:
- M-x gtags-mode
- M-x gtags-find-rtag <a simple kernel symbol, eg printk >

printk: tag not found

Note:
I can successfully search symbols with gtags-find-tag that are inside
kernel tree files from
hello_word.c

---

I have a second observation which is not releated to the previous bug:

- in the documentation for gtags usage is not very clear that GTAGSLIBPATH work
properly _only_ if the four G* files are found somewhere on the path
to the root from
the current point (that is if you are under some "project" files then
GTAGSLIBPATH is follow)
- I use gtags for emacs to browse files and expect gtags to follow
GTAGSLIBPATH to search
other files despite that the four G* files are not present somewhere
on the path to the root.