Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Reduce buffer size from LINE_MAX to PATH_MAX, there is no point to store
path longer than this.
- Fix an unreached case of check against sizeof buf, which in turn leads
to an off-by-one nul byte write on the stack. The original condition
can never be satisfied because the passed boundary is the maximum value
that can be returned, so code was harmless.

MFC after: 1 month

Back out the `hiding' of strlcpy and strlcat. Several people
vocally objected to this safety belt.

`Hide' strlcpy and strlcat (using the namespace.h / __weak_reference
technique) so that we don't wind up calling into an application's
version if the application defines them.

Inspired by: qpopper's interfering and buggy version of strlcpy

Create a small library function, check_utility_compat(3), to determine
whether a named utility should behave in FreeBSD 4.x-compatible mode
or in a standard mode (default standard). The configuration is done
malloc(3)-style, with either an environment variable or a symlink.

Update expr(1) to use this new interface.