libelf: If xlate can only convert the ELF note header, just do that.

libelf: If xlate can only convert the ELF note header, just do that.

When we started parsing new style ELF_T_NHDR8 notes we added extra
checks on alignment and padding. When those failed we would stop
converting and just return the rest of the ELF Note unconverted.
In the case were we just had enough data for just the ELF Note header
and the destionation and source weren't the same we would then
accidentially throw away the Note header conversion we just did.

Fix that by indicating we did correctly convert just the header.

Adds testcase that compares parsing ELF notes with gelf_getnote
and parsing the raw data by hand using elf32_xlatetom using just
the Note header and ignoring the (raw) note data.

Signed-off-by: Mark Wielaard <mark@klomp.org>