Add arbitrary command line flags to llgs/debugserver startup for local debugging.

This patch accepts environment variables of the form:
LLDB_DEBUGSERVER_EXTRA_ARG_n

where n starts with 1, and may continue nearly indefinitely (up through std::numeric_limits<uint32_t>::max()).

The code loops around, starting with 1, until it doesn't find one of the environment variables.  For each one it does find defined, it appends the environment variable's contents to the end of the debugserver/llgs startup command line issued when the stub is started for local debugging.

I am using this to add arbitrary startup commands to the llgs command line for turning on additional logging.  For example:

export LLDB_DEBUGSERVER_EXTRA_ARG_1="-c"
export LLDB_DEBUGSERVER_EXTRA_ARG_2="log enable -f /tmp/llgs_packets.log gdb-remote packets"
export LLDB_DEBUGSERVER_EXTRA_ARG_3="-c"
export LLDB_DEBUGSERVER_EXTRA_ARG_4="log enable -f /tmp/llgs_process.log lldb process"