Clean up iconv/gconv_int.h for unnecessary declarations

The variables __gconv_path_elem, __gconv_max_path_elem_len and function
__gconv_get_path declared in, as well as the type path_elem and macro
GCONV_NCHAR_GOAL defined in gconv_int.h are all used in only one iconv
compilation unit each. In addition, the extern declaration of the variable
__gconv_nmodules refers to a variable that does not exist any more.
Considering this, these symbols do not need to be exposed via a header file.

This patch removes the extern declarations from the header file and moves
the definitions to the compilation units where they are used.