getwd(3) with NULL pointer extension is supported on NetBSD

Summary:
The getwd() and getcwd() functions conform to IEEE Std 1003.1-1990
(POSIX.1).  The IEEE Std 1003.1-2004 (POSIX.1) revision marked
getwd() as legacy and recommended the use of getcwd() instead.  The IEEE
Std 1003.1-2008 (``POSIX.1'') revision removed getwd() from the
specification.

The ability to specify a NULL pointer and have getcwd() allocate memory
as necessary is an extension.

The getwd() function appeared in 4.0BSD.

Reviewers: emaste, tfiala, clayborg

Subscribers: lldb-commits, joerg

Differential Revision: http://reviews.llvm.org/D15260

llvm-svn: 254944