Fix bugs and small changes

Bugs:
- Remove duplicated labels in PackageSpec class;
- Use 'compiler_insensitive' instead of 'compiler' label in Gcc class;
- Caught an unexpected situation when removing packages, but not fully
  fixed;
- Forget to add PACKMAN:: prefix when traping TERM signal;
- Switch subcommand is not process exclusive.

Changes:
- Let ConfigManager report FTP mirror and compiler information;
- Add a 'is_package_installed?' method in install.rb;
- Now install subcommand will call switch subcommand in the end.