tsqa: make failing to write records.config fatal
tsqa: support running tests as root
tsqa: add a simple test for privilege escalation
TS-3103: use scoped ElevateAccess to elevate privileges

Rather than using explicit root privilege escalation, elevate
privilege using the scope ElevateAccess wrapper.
TS-3103: hide ElevateAccess implementation details
TS-3103: improve privilege debug logging

fix better debugging for DebugCapabilities
TS-3103: improve privilege elevation

Remove duplicate user credential routines in favor of a single
ImpersonateUser function. This correctly dealt with real and effective
credentials, supplementary groups and preserving additional process
flags.

Set and preserve PR_SET_PDEATHSIG on Linux so that killing traffic_cop
correctly brings down the rest of the system.

Log and ignore proxy.config.ssl.cert.load_elevated and
proxy.config.plugin.load_elevated unless POSIX capabilities are
enabled.