fix getGroupEntryForID/Name on Solaris

This patch fixes getGroupEntryForID and getGroupEntryForName on Solaris
The issue on Solaris is that it defines both required getgrgid_r
and getgrnam_r functions as CPP macros which depending on configuration
are mapped to real function implementations with different names.
The issue is solved by using C API calling convention instead of platform
C ABI calling convention.