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>

git.c

@@ -83,7 +83,7 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 		 */
 		if (skip_prefix(cmd, "--exec-path", &cmd)) {
 			if (*cmd == '=')
-				git_set_argv_exec_path(cmd + 1);
+				git_set_exec_path(cmd + 1);
 			else {
 				puts(git_exec_path());
 				exit(0);