Split getpwent and fgetgrent functions in interceptors

Summary:
NetBSD does not ship with fgetpwent_r() and fgetgrent_r().
Split their interceptors from getpwent_r() and getgrent_r()
and disable for this OS.

Installation of supernumerary interceptors causes leaking of
errors to dlsym(3)-like operations.

No functional change for other OSes.

Reviewers: vitalybuka, joerg

Reviewed By: vitalybuka

Subscribers: srhines, kubamracek, fedor.sergeev, llvm-commits, #sanitizers, mgorny

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D54041

llvm-svn: 346038