realpath: Improve prev_len logic

[des]

Summary

• Save prev_len after having checked for and appended a trailing slash, not before. This requires us to back up if we end up returning a partial result, but previously we would sometimes return a partial result with a trailing slash and sometimes without.

• Replace strlcat() with a faster strlcpy() since we know exactly how far into the buffer we are.

MFC after: 1 week
Sponsored by: Klara, Inc.

[des]

fix

[kevans]

lib/libc/stdlib/realpath.3

84 ↗

(On Diff #173850)

I'd be tempted to rephrase this just a little more, because right now the implied part is the important part:

or a pointer to a null-terminated string that the caller must
.Xr free 3
if it was.

[des]

I'm also tempted to drop the CAVEATS section, which does not seem useful as realpath(3) was standardized by POSIX in 1994 and I'm sure Solaris / Illumos also always returns an absolute path.

[des]

Split man page changes off into D55928