Fixed includes.
`eiki_install_signal_handler()` catches SIGABRT.

Make `eiki_install_signal_handler()` catch SIGABRT as well. Added
`eiki_abort()` to re-register the default signal handler for SIGABRT and
then call `abort()`; this is safe since both `signal(2)` and `abort(3)`
are async-signal-safe. This is useful for allowing Eiki to print a
parting stack trace when aborting; notably, glibc's `assert()` calls
`abort()` on an assertion failure.
`eiki_print_signal()` prints memory address.

Changed `eiki_print_signal()` to also print the faulting memory address
if available. While this is usually redundant with the stack trace, it
may be useful if for whatever reason the stack trace is unavailable.