run_command(): handle missing command errors more gracefully

When run_command() was asked to run a non-existant command, its behavior
varied depending on the platform:

  - on POSIX systems, we would fork, and then after the execvp call
    failed, we could call die(), which prints a message to stderr and
    exits with code 128.

  - on Windows, we do a PATH lookup, realize the program isn't there, and
    then return ERR_RUN_COMMAND_FORK

The goal of this patch is to make it clear to callers that the specific
error was a missing command. To do this, we will return the error code
ERR_RUN_COMMAND_EXEC, which is already defined in run-command.h, checked
for in several places, but never actually gets set.

The new behavior is:

  - on POSIX systems, we exit the forked process with code 127 (the same
    as the shell uses to report missing commands). The parent process
    recognizes this code and returns an EXEC error. The stderr message is
    silenced, since the caller may be speculatively trying to run a
    command. Instead, we use trace_printf so that somebody interested in
    debugging can see the error that occured.

  - on Windows, we check errno, which is already set correctly by
    mingw_spawnvpe, and report an EXEC error instead of a FORK error

Thus it is safe to speculatively run a command:

  int r = run_command_v_opt(argv, 0);
  if (r == -ERR_RUN_COMMAND_EXEC)
	  /* oops, it wasn't found; try something else */
  else
	  /* we failed for some other reason, error is in r */

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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