Debugger: Fix some broken comparisons.

* Missing parentheses in integer comparison inversion
* !=, not ! at the beginning of expression

Spotted by Clang.

debugger/elf: Don't leak memory in failure case.

CID 1423098.

libdebugger: Fix a regression with ELF loading in some cases.

ElfFile:
- The refactoring in commit 2c50fbb8e16102886d0cd28135dae0b58b9d33f4
inadvertently introduced a failure case if a section in the ELF file
is skipped, since it was explicitly passing in the insertion index
to BObjectList::AddItem(). Since in the skipped case, this would be
out of bounds for the list's current size, this would cause the whole
operation to fail with an out of memory error, and consequently halt
all further loading of debug information for that particular file.
This manifested itself with libroot on at least gcc5, where no symbols
could be seen.

elf.h: rename ELF_MAGIC to ELFMAG

- This is how it is named in other versions of elf.h (Linux, glibc, possibly more)
- ELF_MAGIC is used by libelf for the same thing, and the defines conflicts,
breaking libelf build on Haiku.

Debugger: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.