Prevent launch process from returning until first stop.

Prevent the $A launch command from returning to the gdb-remote client
until after the initial stop is received.  This eliminates a race on
the client starting to communicate with the inferior before the process
is ready.

See https://github.com/tfiala/lldb/issues/24

This is not the whole story - I am still getting the initial disassembly
dump intermittently, and it appears to come from the startup location
of the app (i.e. where libc comes into play).