malloc: Replace shell/Perl gate in mtrace

The previous version expanded $0 and $@ twice.

The new version defines a q no-op shell command.  The Perl syntax
error is masked by the eval Perl function.  The q { … } construct
is executed by the shell without errors because the q shell function
was defined, but treated as a non-expanding quoted string by Perl,
effectively hiding its context from the Perl interpreter.  As before
the script is read by require instead of executed directly, to avoid
infinite recursion because the #! line contains /bin/sh.

Introduce the “fatal” function to produce diagnostics that are not
suppressed by “do”.  Use “do” instead of “require” because it has
fewer requirements on the executed script than “require”.

Prefix relative paths with './' because “do” (and “require“ before)
searches for the script in @INC if the path is relative and does not
start with './'.  Use $_ to make the trampoline shorter.

Add an Emacs mode marker to indentify the script as a Perl script.