ovs-bugtool: Restore RHEL support.

ovs-bugtool: Restore RHEL support.

ovs-bugtool was originally xen-bugtool from Citrix XenServer.  We modified
it for OVS by tailoring it to work better on Debian, updating file
locations and removing features that were specific to XenServer or that
work with packages not installed by default on Debian.

This commit reverts many of those changes.  This commit:

  - Adds back code that works with RHEL features not installed by default
    on Debian (but not XenServer-specific features, since xen-bugtool works
    nicely on XenServer).

  - Switches from hard-coded paths for utilities to searching the
    superuser's typical $PATH (because RHEL and Debian disagree on the
    location for some utilities).

  - In a few cases merges lists, e.g. now it looks for logs under the names
    used in both Debian and RHEL.

  - Fixes a few spurious differences between ovs-bugtool and xen-bugtool,
    e.g. in white space.