libelf: add elf_check_magic function

libelf: const-qualify strings

Strings returned by this library should be considered read-only.
If string tables are to be modified, elf_getSection should be used.

libelf: remove duplicate and add missing functions

By normalising return types, most functions in `elfxx.c` can be merged and
moved into `elf.c`.

Also add missing prototypes and functions for accessing ELF fields.
- getSectionStringTableIndex
- getSectionNameOffset
- getSectionOffset
- getSectionLink
- getSectionInfo
- getSectionAddrAlign
- getSectionEntrySize

libelf: normalise types

Use size_t and uintptr_t where appropriate.
Add enum elf_addr_type for physical/virtual memory.

libelf: introduce memory safety

The size of the buffer containing the ELF file needs to be known to
access it safely.

Change all public void * references to elf_t *, which requires a size
parameter.

libelf: make elfxx.h internal

libelf: re-order functions

Re-order functions based on purpose and ELF header location.

libelf: delete unnecessary functions

This deletes:

elf_getProgramHeaderInfo: This is an awkward convenience function that
appears to be unused.

elfxx_getProgramSegmentTable,
elfxx_getSegmentType: Functions with different names but the same
functionality already exist.

elfxx_getSegmentInfo: Both of the above.

libelf: normalise indents and whitespace

libelf: remove debug functions

These functions have not been maintained and are out of date.

pragma once and for all

SELFOUR-1083 optionally return section ID in elf_getSectionNamed

libelf: Implement helpers for retrieving alignment

The program headers have helpers for retrieving every value except the alignment,
this commit adds a helper for that.

libelf: Retrieve ELF constants from underlying C library

Use the System V defined ELF extension header to retrieve the ELF format constants. Other
parts of this library could also be changed to use definitions from the standard ELF header,
but these are the ones that generate name collisions when using both this library and the
ELF header concurrently.

Fix whitespace

- remove trailing whitespace
- remove duplicate blank lines
- remove blank lines at end of file

Remove trailing whitespace

libelf: Qualify some const char * parameters.