Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Add missing static keywords to csplit(1)

Use fseeko() to properly support large files.

Use errc().

The return value of snprintf should be always nonnegative, so it should
be safe to cast it to a size_t before comparing it to a sizeof().

Use sigaction(2) instead of signal(3) to avoid the signal handler being
re-entered.

Avoid truncating filenames with snprintf().

Respect LC_CTYPE and LC_COLLATE environment variables, mainly for their
effects on regular expressions. The libc regex code does not handle
equivalence classes well (for example), but we do our best.

Handle the case where a negative offset in a regular expression match
is larger than the number of lines in the overflow file.

Correct synopsis and usage(): pattern arguments are not optional.

Add the csplit(1) utility, which splits files based on context, as specified
by SUSv3.

PR: 36191
Reviewed by: mike