ash: add support for command_not_found_handle hook function (bashism)

ash: add support for command_not_found_handle hook function (bashism)

This implements support for the command_not_found_handle hook function, which is
useful for allowing package managers to suggest packages which could provide the
command.

Unlike bash, however, we ignore exit codes from the hook function and always return
the correct POSIX error code (EX_NOTFOUND).

function                                             old     new   delta
find_command                                         911     990     +79

Signed-off-by: William Pitcock <nenolod@dereferenced.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>