exec_cmd: RUNTIME_PREFIX on some POSIX systems

Enable Git to resolve its own binary location using a variety of
OS-specific and generic methods, including:

- procfs via "/proc/self/exe" (Linux)
- _NSGetExecutablePath (Darwin)
- KERN_PROC_PATHNAME sysctl on BSDs.
- argv0, if absolute (all, including Windows).

This is used to enable RUNTIME_PREFIX support for non-Windows systems,
notably Linux and Darwin. When configured with RUNTIME_PREFIX, Git will
do a best-effort resolution of its executable path and automatically use
this as its "exec_path" for relative helper and data lookups, unless
explicitly overridden.

Small incidental formatting cleanup of "exec_cmd.c".

Signed-off-by: Dan Jacques <dnj@google.com>
Thanks-to: Robbie Iannucci <iannucci@google.com>
Thanks-to: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0061-run-command.sh

@@ -145,7 +145,7 @@ test_trace () {
 	expect="$1"
 	shift
 	GIT_TRACE=1 test-tool run-command "$@" run-command true 2>&1 >/dev/null | \
-		sed 's/.* run_command: //' >actual &&
+		sed -e 's/.* run_command: //' -e '/trace: .*/d' >actual &&
 	echo "$expect true" >expect &&
 	test_cmp expect actual
 }