MFC r358512: Move ELF feature note tool to usr.bin/elfctl

MFC r358512: Move ELF feature note tool to usr.bin/elfctl

elfctl is a tool for modifying the NT_FREEBSD_FEATURE_CTL ELF note,
which contains a set of flags for enabling or disabling vulnerability
mitigations and other features.

Also merge follow-on commits:

r358518 elfctl: initialize features
r358546 elfctl: tiny style(9) cleanup, use bool where appropriate
r358622 elfctl: style(9): use C99 uintX_t types
r358623 elfctl: check read return value
r358889 elfctl: remove memory leak

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