Add user and system install, uninstall and symlink

[trufae]

This PR changes the behaviour of the make install procedure

in master:

• make install : copies the plugin in your home

in this branch:

• make install: copies the plugin and libdec into your system plugin directories
• make user-install: same as above, but in your home.
• make symlink: symlinks the plugin and libdec

The reason behind this change is to make the makefile behaviour more consistent with other r2 plugins as well as fixing the debian package which was missing some files and the R2DEC_HOME path set at compile time wasn't working even if setting the env var. With this PR the generated .deb works.

[wargio]

looks like the makefile has some errors