MFC r358891, r358894, r359454, r359513, r359550 and r359556

elf(5) Updates:

• start documenting ELF note sections
• Remove extra Sy from table header
• table markup fixes
• add definitions of FreeBSD notes
• minor update to FreeBSD ELF note descriptions
• correct and expand NT_FREEBSD_NOINIT_TAG description
• expand on NT_FREEBSD_FEATURE_CTL bit definitions

Reviewed by: emaste
Approved by: emaste