unix: use proper signal names instead of numbers
unix: use proper signal names instead of numbers

In POSIX standard only several signals have assigned numbers, see [0].
'0' is EXIT, '3' is QUIT, '15' is TERM. '4' is not assigned in POSIX.
On Linux, FreeBSD, and Mac OS X '4' is assigned to ILL, which stands for
'Illegal instruction', see [1], par. 7.14 of [2]. Given that C-compilers
tend to produce valid instructions there's little point in trapping ILL.

Thus trap INT, QUIT, TERM, and EXIT signals. Note that INT replaced ILL.

[0]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#trap
[1]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/signal.h.html
[2]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

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