Signal improvements and fixes

There were few issues in zm_signal:
1) The addr2line path was wrong, It showed this:
addr2line -e /home/kfir/zmwork/ZoneMinder/src/zmc() 0x7ffff7172500 0x42d5c8 0x7ffff4627cdd 0x42cc79
Instead of this:
addr2line -e /home/kfir/zmwork/ZoneMinder/src/zmc 0x7ffff7172500 0x42d5c8 0x7ffff4627cdd 0x42cc79
2) The checking and including of the ucontext header was not properly implemented.
3) No support for 64bit instruction pointer. It was only checking for EIP, but not RIP.
4) Displaying backtrace depended on the availability of the sigcontext or siginfo & ucontext structs.
5) No check for the function backtrace_symbols.
6) Not including execinfo.h only when available, as this is a GNU extension, just like backtrace and backtrace_symbols.
7) No handling for Ctrl+C (SIGINT)
8) sigcontext is undocumented and now deprecated, and shows an invalid instruction pointer when used in ZM

I have made the code more readable, and also used indent -linux to clear the mix of few lines using spaces, while others are using tabs. It converted the whole file to use tabs and did appearance changes to the file