Stop assuming .text and .eh_frame segment

The coredump remote was architected on the assumption that the .text and
.eh_frame sections were mapped onto the same segment, and that that segment was
always the first PT_LOAD segment in an ELF file. Well, that was never a valid
assumption, and moderns releases of various toolchains have started splitting
the PT_LOAD segments for security reasons.

This change implements an M:N mapping of PT_LOAD segments in a coredump file to
backing ELF files and calculates and adjusts offsets appropriately. Because the
backing files get mapped in a lot of file I/O operations have been replaced with
simple memory reads. Once a backing file is memory mapped is stays mapped until
the address space is destroyed.

The ucd_*.[ch] files contain only functions that should not be exposed through
the public API so they;re not mangled using the UB naming schedule because I
just bring myself to write code with undefined behaviour.

Reformatted some of the changed files using `astyle --style=gnu` for internal
consistency withing the file.

Fixes #363

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