Two fixes regarding plugin management

Fixed plugin reload use-after-free

  There was a use-after-free bug due to plugin_list not being re-initialized - to NULL - once the plugin list is entirely freed.
  


Fixed perl VM double-termination (through PERL_SYS_TERM()'s double-call)

  PERL_SYS_TERM() was called at each perl-plugin reload (using the `r' key) whereas perlembed(1) precising the following:
"The macros PERL_SYS_INIT3() and PERL_SYS_TERM() provide system-specific tune up of the C runtime environment necessary to run Perl interpreters; they should only be called once regardless of how many interpreters you create or destroy".
  PERL_SYS_INIT3() was also added in this commit, it shouldn't be a problem there called once there.

Open to discussion :)