fix spurious errors in refactored passwd/group code

errno was treated as the error status when the return value of getline
was negative, but this condition can simply indicate EOF and is not
necessarily an error.

the spurious errors caused by this bug masked the bug which was fixed
in commit fc5a96c9c8aa186effad7520d5df6b616bbfd29d.

refactor passwd file access code

this allows getpwnam and getpwuid to share code with the _r versions
in preparation for alternate backend support.

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.

make passwd/group functions safe against cancellation in stdio

these changes are a prerequisite to making stdio cancellable.

initial check-in, version 0.5.0