opts: fix CURLOPT_UNIX_SOCKET_PATH formatting

Add .nf and .fi such that the code gets wrapped in a pre on the web.
Fixed grammar, fixed formatting of the "See also" items.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
tool: fix CURLOPT_UNIX_SOCKET_PATH in --libcurl output

Mark CURLOPT_UNIX_SOCKET_PATH as string to ensure that it ends up as
option in the file generated by --libcurl.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>