libbacktrace: support fetching executable name using sysctl

libbacktrace: support fetching executable name using sysctl

This supports FreeBSD and NetBSD when /proc is not mounted.

libbacktrace/
	* fileline.c (sysctl_exec_name): New static function.
	(sysctl_exec_name1): New macro or static function.
	(sysctl_exec_name2): Likewise.
	(fileline_initialize): Try sysctl_exec_name[12].
	* configure.ac: Check for sysctl args to fetch executable name.
	* configure: Regenerate.
	* config.h.in: Regenerate.