extdiff: fix defaulting to "diff" if no --program is given

dirstate: add/improve method docstrings. - add doc to __init__(), _map(), status() - enhance for __getitem__() - fix inaccurate doc for walk() (described wrong return type)

util.system: Use subprocess instead of os.system subprocess allows the environment and working directory to be specified directly, so the hacks for making temporary changes while forking is no longer necessary. This also fixes failures on solaris where the temporary changes can't be undone because there is no unsetenv.