README.extern, tests, and a bug fix

[Reviewed by Lydia, David, Tom, and Greg]

I set about updating README.extern with the newly implemented
c_string and c_ptr(type). In doing so, I constructed some example
code, so of course I created tests with the example code. Through
this exercise I discovered a bug with c_string on multilocale
configurations - declaring var x:c_string; would in some cases
try to 'put' &NULL to a remote c_string. I solved the
problem by making the default initializer for c_string the
symbol chpl_emptyCstring which is now #defined in chpltypes.h to "".

In README.extern, did a better job describing the tricky bits of extern
blocks and described c_ptr and c_string types in detail.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21979 3a8e244f-b0f2-452b-bcba-4c88e055c3ca