Split main/chpl_main into intialization functions.

Motivated by Chapel "library-mode", next step is splitting "chpl_gen_main".
Added 'chpl-init.h' to headers.
Removed postUserCodeHook from library mode.
Added -fPIC to build of runtime.
Added check for CHPL_LIBMODE env-var to enable "-fPIC" compilation flag.
Merge branch 'master' of github.com:safl/chapel