[v2.3.11] New subcommand "get -expect-buf N": dump data in the internal expect buffer

[clarkwang]

... for easy debugging when writing sexpect scripts.

[clarkwang]

get <-expect-buffer COUNT | -expbuf COUNT>

• COUNT: [1, 4096]
• Use -cstring format ^{(implying that the output can be safely passed to expect -cstring -exact "string")}.
• Don't pass COUNT to server side. The client side deals with it.

[clarkwang]

Fixed in commit 0febb18