Replace by a copy of glibc/sysdeps/posix/getcwd.c, but

Replace by a copy of glibc/sysdeps/posix/getcwd.c, but
with the following changes.
(__set_errno): Parenthesize properly.
Include <stdbool.h>.
(MIN, MAX, MATCHING_INO): New macros.
(__getcwd): Define with prototype, not K&R form.
Use heuristics to allocate default buffer on stack if possible.
If AT_FDCWD is defined, use openat and fstatat to avoid O(N**2)
behavior, and to avoid the PATH_MAX limit when computing
../../../../...
Use MATCHING_INO to compare inode number to file.
Check for arithmetic overflow in size calculations.
Fix bug in reallocation of dot array that caused getcwd to fail
on directories nested deeper than 75.
Be more careful about saving errno on error.
Do not use realloc; use only free+malloc, as this is a bit
more flexible and avoids a needless copy operation.
Do not inspect st_dev and st_ino for symbolic links; POSIX
doesn't specify the latter.
Check for closedir errors.
Avoid needless casts.
Use "#ifdef weak_alias" around weak_alias, to be like other
glibc code.
The following changes to getcwd.c have effect only when used in
gnulib; they have no effect inside glibc proper.
(#pragma alloca) [defined _AIX && !defined __GNUC__]: Remove,
as alloca isn't used.
(alloca, __alloca): Likewise.
[!_LIBC]: Include "getcwd.h", "mempcpy.h".
Include <stddef.h>, <stdlib.h>, <string.h>, <limits.h>
unconditionally, as gnulib assumes C89 or better.
Do not include <sys/param.h>.
(errno) [!defined __GNU_LIBRARY__ && !defined STDC_HEADERS]: Remove
no-longer-necessary 'extern int errno' decl; gnulib assumes C89 or
better.
(NULL) [!defined NULL]: Remove; we assume C89 or better.
Include <dirent.h> in a way that is compatible with modern Autoconf.
(_D_ALLOC_NAMELEN, _D_EXACT_NAMLEN):
New macros, if not already defined.
Include <unistd.h> if _LIBC, not if __GNU_LIBRARY__.
Use "_LIBC", not "defined _LIBC", for consistency.
(HAVE_MEMPCPY): Remove; no longer needed now that gnulib has
a mempcpy module.
(__lstat, __closedir, __opendir, __readdir) [!_LIBC]: New macros.
(GETCWD_RETURN_TYPE): Remove.  All uses replaced by char *.