fix state directory crash

If a state directory was defined as relative subdirectory the command
crashed.

$ f5gs --state foobar
Segmentation fault.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
add few basic regression tests

Most notably script running is not tested, but it is a tough one to crack
without changing script directory.  It is probable I need to create
another version of command with compiler #ifdef and check_PROGRAMS that
allows script directory overriding.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
add pid file path to error message

Didn't I already do this in commit be7979b?  Oh well, maybe I did miss
this message earlier.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
remove obsolete gnulib module

Signed-off-by: Sami Kerola <kerolasa@iki.fi>