[lldb-mi] Add support for StopAtEntry in MI via "-exec-run --start".

This patch adds a --start option to the lldb-mi -exec-run command for
getting process stopped at entry point after launch.  It is equivelent
to the -s option in the lldb command line interpreter:
    process launch -s
and is therefore not supported on all hosts and/or targets.  To check
if the --start option is supported, see if the corresponding feature
"exec-run-start-option" is in the list of options reported by the lldb-mi
"-list-features" command.

Patch from engineer.developer@gmail.com (Kirill Lapshin)
Reviewed by: ki.stfu
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D12977

llvm-svn: 249072