Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Rename libohash to libopenbsd.

libopenbsd is an internal library which
to bring in compatibility stuff from OpenBSD.
This will allow us to bring in more
OpenBSD utilities into the FreeBSD base system.

We similarly use libnetbsd for bringing in stuff from NetBSD.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D4078

Include stddef.h for ptrdiff_t

Make libohash a proper internallib instead of hidding it in m4

mandoc development seems to be also using ohash so prepare to share the code when
needed

Fix build with gcc

Sync with OpenBSD
This brings:
- check for integer overflows in custom allocs
- fix potential integer overflows in memory allocation
- annotate regexp error messages with source string
- better error handling in mkstemp/unlink/fdopen logic

Synchronize with laster version of m4 from OpenBSD and NetBSD
This bring better compatibility with gnum4

Reviewed by: cognet
Approved by: cognet
Obtained from: OpenBSD, NetBSD