Added "library-mode" / CHPL_LIBMODE for compiling Chapel into (shared) libraries.

 * Split runtime initialization into chpl_rt_init / chpl_rt_finalize.
 * Added functions:
    - chpl_library_init / chpl_library_finalize used by libraries to
      initialize / finalize runtime.
    - chpl_executable_init / chpl_executable_finalize used by executables
      to initialize/finalize runtime.
 * Makefile check for CHPL_LIBMODE=shared, adding "-fPIC" flag for runtime.

Most of the code that was previously runtime/src/main.c is now in
runtime/src/chpl-init.c, primarily most of what used to be
"main(...)" is now "chpl_rt_init(...)".

Most of what used to "chpl_main" is now in "chpl_std_module_init", also
"chpl_main" is no more, instead "chpl_executable_init" is used by
"main(...)".
Fixed return-type of chpl_library_init.
Fixed missing prototype for 'chpl_std_module_init'.
Removed the "functionally empty" file "chpl-main.h".
Contents of "chpl-main.h" is now in "chpl-init.h".
Merge pull request #945 from safl/libmode

Added "library-mode" / CHPL_LIBMODE for compiling Chapel into (shared) libraries.