Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Document that the *fsize() functions return a size of 1 for Elf
types that don't have a fixed size.
- The *fsize() functions should return a size of 1, for variable length
types.
- Redefine symbol ELF_T_LAST to match the current end of the list.

Obtained from: elftoolchain
MFC after: 1 month

Reduce verbosity.

Obtained from: elftoolchain
MFC after: 1 month

- Return zero for file sizes of ELF types that have a variable size.
- Neaten a few comments.

Obtained from: elftoolchain
MFC after: 1 month

Changes for supporting GNU Hash sections.

Obtained from: elftoolchain
MFC after: 1 month

Use <osreldate.h> rather than the OS version reported by the kernel
to determine which Elf Types are to be handled. Change the M4
templates to wrap an `#if __FreeBSD_version >= NNN'/`#endif' pair
around the generated code for each ELF data type, where `NNN' is
the OS version where the ELF type was added to the source tree.

This change allows cross-builds of old sources on newer FreeBSD
systems to work correctly.

Problem reported by: ru

MFP4: Add an implementation of the ELF(3) and GELF(3) API set.

Bump __FreeBSD_version.

Reviewed by: jb