Prior to this fix, when completing a command that doesn't have a /, we
would prepend each component of PATH and then expand the whole thing. So
any special characters in the PATH would be interpreted when performing
tab completion.
With this fix, we pull the PATH resolution out of complete.cpp and
migrate it to expand.cpp. This unifies nicely with the CDPATH resolution
already in that file. This requires introducing a new expand flag
EXPAND_SPECIAL_FOR_COMMAND, which is analogous to EXPAND_SPECIAL_CD
(which is renamed to EXPAND_SPECIAL_FOR_CD). This flag tells expand to
resolve paths against PATH instead of the working directory.
Fixes#952
Directories are completed like commands, because of implicit cd.
However, directories found inside $PATH entries should not be completed,
as implicit cd doesn't work there. Similarly, directories should not be
completed after the `command` builtin.
Fixes#1695.
