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

Teach libdwarf about the DW_FORM_flag_present dwarf attribute, so
programs using libdwarf (such as ctfconvert) don't error out on files
containing the attribute.

MFC after: 2 weeks

Add a BSD licensed DWARF library for use by the DTrace clients.

The API for this library is deliberately different to the GPL'd
libdwarf to avoid licensing problems.