Fix an occasion in mirror subcommand

In some occasion, the pid file of proftpd is gone, but proftpd deamon
is still on. So improve the status_mirror_service and stop_mirror_service
methods to handle this occasion.
Merge branch 'master' of github.com:dongli/packman
Update package DSL

Now we should use more concise option usage in package classes as:

- 'use_mpi?': whether MPI codes are turned on;
- 'with_fortran?': whether Fortran bindings are turned on;
- 'mpi': if MPI codes are turned on, what MPI implementation do we use.

Add Netcdf master package to enclose C, C++ and Fortran bindings in one
installation directory.