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 common Unix library way of definig wrappers
for those functions in HsUnix.c and this way CPP will do its job correctly