Renamed the "commands" command to "command" as this is the way we were using

it everywhere internally.

Modified the "command regex" command to be able to specify all regular 
expressions on the command line. For example:

(lldb) command regex f s/^$/finish/ 's/([0-9]+)/frame select %1/'

Also improved the error reporting when an invalid 's/<regex>/<subst>/' argument
is given.

llvm-svn: 129889