Handle FreeBSD-specific ELF notes

Add a function to return the specific type, when the note's Name field is
'FreeBSD'.

r249558 added FreeBSD-specific ELF note types that reuse type numbers of
existing generic / Linux types.  This caused 'readelf -n' to produce
incorrect output on FreeBSD core files.

Sponsored by:	DARPA, AFRL
MFC after:	3 days