extcmd based on glibc must respect the on_line() protocol

Old thread based extcmd was doing line-by-line reads with internal buffering. New extcmd is doing block-based reads which are not bound by the end-of-line constraints. This breaks applications that always expect to have at most one line to process.

For example (excuse the translation here), here the rfc822 parser cannot handle a multi-line buffer correctly:

DEBUG plainbox.secure.rfc822: Patrzenie na linię 1:'id: DEBUG\n\nid: plainbox.clitools:\n\nid: Activated\n\nid: early\n\nid: logging\n\nid: via\n\nid: environment\n\nid: variables\n\nid: DEBUG\n\nid: plainbox.clitools:\n\nid: Constructed\n\nid: <plainbox.impl.box.PlainBoxTool\n'

Before the interface offers buffer-based events, extcmd must emulate the old behavior.