fix crashes in refactored passwd/group code

the wrong condition was used in determining the presence of a result
that needs space/copying for the _r functions. a zero return value
does not necessarily mean success; it can also be a non-error negative
result: no such user/group.

refactor group file access code

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

fix off-by-one error in getgrnam_r and getgrgid_r, clobbering gr_name

bug report and patch by Michael Forney. the terminating null pointer
at the end of the gr_mem array was overwriting the beginning of the
string data, causing the gr_name member to always be a zero-length
string.

more close-on-exec fixes, mostly using new "e" flag to fopen

make passwd/group functions safe against cancellation in stdio

these changes are a prerequisite to making stdio cancellable.

it's called getgrgid_r, not getgruid_r...

initial check-in, version 0.5.0