Add tool to modify ELF binary feature control bits

MFC r352797: Add tool to modify ELF binary feature control bits

This will allow feature control bits (e.g. for ASLR, PROT_MAX) to be
inspected or modified.

Some clean-up and additional work is likely still required, but we can
iterate on this in the tree.

Submitted by: Bora Özarslan <borako.ozarslan@gmail.com>

MFC r352799: controlelf: clean up warnings

• use explicit ELF note name when not found
• no trailing . on warnings
• no \n

MFC r352801: controlelf: install standard BSD 2 clause license

MFC r352803: controlelf: some style(9) cleanup

MFC r352805: controlelf: add protmax control

MFC r352806: controlelf: tidy up option parsing

MFC r352808: controlelf: simplify feature string parsing

Also add error handling on failure to seek/write updated value.

MFC r352809: controlelf: exit with error if file endianness does not match host

We need to add support for cross-endian operation, but until that's done
just exit with an error rather than misbehaving.

MFC r352815: controlelf: update man page

Some minor corrections, clarifications or rewording.

Sponsored by: The FreeBSD Foundation