exec_cmd.c: replace hard-coded path list with one from <paths.h>

exec_cmd.c: replace hard-coded path list with one from <paths.h>

The default executable path list used by exec_cmd.c is hard-coded to
be "/usr/local/bin:/usr/bin:/bin".  Use an appropriate value for the
system from <paths.h> when available.

Add HAVE_PATHS_H make variables and enable it on Linux, FreeBSD,
NetBSD, OpenBSD and GNU where it is known to exist for now. Somebody
else may want to do an autoconf support later.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>