MFC r249377, r249378, r249379:

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.

git-svn-id: svn://svn.freebsd.org/base/stable/9@250308 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f