signals: don't trap segfault/abort when envvar NANO_NOCATCH is defined

When the user wants to get a backtrace, the crash handler shouldn't
kick in.

doc/nano.1

@@ -309,6 +309,10 @@ not by default distinguish modified from unmodified arrow keys.
 Please report any other bugs that you encounter via:
 .br
 \fIhttps://savannah.gnu.org/bugs/?group=nano\fR.
+.sp
+When nano crashes, it will save any modified buffers to emergency .save files.
+If you are able to reproduce the crash and you want to get a backtrace, define
+the environment variable \fBNANO_NOCATCH\fR.
 
 .SH HOMEPAGE
 \fIhttps://nano-editor.org/\fR

src/nano.c

@@ -1213,6 +1213,7 @@ void signal_init(void)
 	}
 
 #ifndef DEBUG
+	if (getenv("NANO_NOCATCH") == NULL) {
 	/* Trap SIGSEGV and SIGABRT to save any changed buffers and reset
 	 * the terminal to a usable state.  Reset these handlers to their
 	 * defaults as soon as their signal fires. */
@@ -1220,6 +1221,7 @@ void signal_init(void)
 	act.sa_flags |= SA_RESETHAND;
 	sigaction(SIGSEGV, &act, NULL);
 	sigaction(SIGABRT, &act, NULL);
+	}
 #endif
 }