pkg ('rebuild') errors out

[Thomas Weber]

Hi, 

on a freshly installed Octave installation (3.0.4rc3, no packages ever
installed), the following happens:
===========================================================================
octave:1> pkg ('rebuild')
error: couldn't read directory /usr/local/share/octave/packages: No such
file or directory
error: evaluating if command near line 427, column 5
error: evaluating if command near line 425, column 3
error: called from `pkg:rebuild' in file
`/usr/local/share/octave/3.0.4/m/pkg/pkg.m'
error: evaluating assignment expression near line 374, column 18
error: evaluating if command near line 373, column 7
error: evaluating switch command near line 272, column 3
error: called from `pkg' in file `/usr/local/share/octave/3.0.4/m/pkg/pkg.m'
===========================================================================

This generates problems in distributions (at least in Debian) later:
when all packages are removed from a system, the package manager assumes
that no package is using /usr/share/octave/packages/ and removes that
empty directory.

I see two ways out: 
1) Octave should provide the packages/ directory
2) pkg.m should not error out when asked to rebuild with a non-existant
package directory.

I think 1) makes more sense and is probably easier to implement.

	Thomas