Fix arc keymaster bootstrapping with ipcz

Arc keymaster code uses the Mojo invitations API to bootstrap multiple
pipes to the keymaster server. It is the only known client which uses
invitations the way that it does -- i.e., attaching multiple
string-named pipes -- and this usage is not supported by MojoIpcz.

The bootstrap code also does not account for the possibility of
attachment failure, so this can lead to further bugs after attachment
fails.

This CL changes keymaster bootstrapping to use different (numeric)
attachment names for the pipes when MojoIpcz is enabled, and it
introduces graceful handling of attachment failures in case of
future breakage.

Fixed: 1379831
Change-Id: I11c6d5d6e4c3d9bf9bca955ad2ff21567ab66af2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3995204
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1066341}