Fix build on recent glibc

[nyanpasu64]

This PR removes the usage of the sys_siglist, which was removed in glibc 2.32, causing this project to fail to build on recent Linux distros.

I based my approach off an existing project's bugfix (replace sys_siglist[sig] with strsignal(sig)). The documentation for the removal of sys_siglist is in the glibc 2.32 release notes (2020-08).

The usage of sys_siglist in this program is very minimal, so in-depth analysis is probably overkill. But I looked deeper anyway...

Looking at the glibc 2.32 release notes, it appears there are multiple functions returning different types of names, but strsignal is good enough for this case.

Looking at the strsignal manpage... sigh it's not thread-safe, and the return value may be null for unrecognized signals on OSes other than Linux. It doesn't matter in this case though, since we're on Linux and hopefully all signals are recognized.

I found a source saying that there are no known OSes with sys_siglist but not strsignal, so this change should not reduce compatibility with any versions of Linux.

(I was unable to make debugme actually work on Arch Linux though. gdb never triggered when an app segfaulted. Perhaps drkonqi is interfering?)

Fixes #3.

[codecov-commenter]

Codecov Report

Merging #4 (9e2a615) into master (2171981) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #4   +/-   ##
=======================================
  Coverage   58.15%   58.15%           
=======================================
  Files           4        4           
  Lines         141      141           
=======================================
  Hits           82       82           
  Misses         59       59           

Impacted Files	Coverage Δ
src/debugme.c	69.76% <100.00%> (ø)

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