nscd step6

[david_crossfamilyweb.com]

Summary

minor code quality fixups for getpwent.
implement libc side of grouplist and pivot lookups for caching

[markj]

lib/libc/gen/getgrent.c

209

370

original_ngroups is an "untrusted" value, i.e., it comes from nscd. This code should at least verify that

1. it's non-negative,
2. that original_ngroups * sizeof(gid_t) won't overflow,

before doing math with it.

[david_crossfamilyweb.com]

Implemented all PR changes