Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r249377, r249378, r249379:

r249377:

Add sbuf_start_section() and sbuf_end_section() functions, which can
be used for automatic section alignment.

Discussed with: kib
Reviewed by: kib

r249378:

Add sbuf_start_section and sbuf_end_section to the libsbuf symbol map.

r249379:

Document sbuf_start_section() and sbuf_end_section() functions.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Ok, so we use different comment conventions in Symbol.map and Version.def
"#notimpressed" as my twittering friends would say.

Bump shlib version
Instigate symbol versioning
Pray that I don't break the build.