Fix three Windows subprocess issues

Summary:
Fixes T13504. This fixes three issues:

1. In ExecFuture, "proc_open()" on an invalid binary could fail with an unconditional exception.
2. In ExecPassthru, "proc_open()" on an invalid binary could fail with an unconditional exception.
3. In "arc browse", "start <url>" does not work when the shell is bypassed.

In (1) and (2), the desired behavior is to fail with an exit code which is sometimes upgraded to an exception depending on calling convention.

Issue (1) most commonly manifested as "find" failing when run via "cmd.exe".

Issue (2) most commonly manifested as "arc browse" failing.

Issue (3) was entangled with issue (2).

In cases (1) and (2), assume "proc_open()" failures under Windows are because of bad binaries and treat them like bogus commands on Linux/Mac.

In case (3), use "cmd /c start" instead of "start" as a default browser on Windows.

Test Plan:

• On Windows, did mime type detection in cmd.exe. Before patch: proc_open() exception in "find". After patch: clean (albeit not terribly useful) detection.
• On Windows, did "arc browse ...". Before patch: proc_open() exception in "start". After patch: clean browser execution.

Maniphest Tasks: T13504

Differential Revision: https://secure.phabricator.com/D21047