user-mgr: convert boost::process::child to fork/execv

Boost process v1 is deprecated and removed in the next version
of Boost.  process v2 is async-only and cannot easily be replaced
with the current usages.  Switch to use raw fork/execv calls instead,
which is what boost would have done internally.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia802cc5428b37dbe428bf1f8befcac168ff3262b
2 files changed