change uid_t, gid_t, and id_t to unsigned types

change uid_t, gid_t, and id_t to unsigned types

this change is both to fix one of the remaining type (and thus C++
ABI) mismatches with glibc/LSB and to allow use of the full range of
uid and gid values, if so desired.

passwd/group access functions were not prepared to deal with unsigned
values, so they too have been fixed with this commit.