Updates to bin/clean due to SVN to Git repository conversion.

Git's '.gitignore' files are more sophisticated than CVS's
'.cvsignore' files and SVN's 'svn:ignore' properties.  In particular,
Git's '.gitignore' patterns are cumulative from the root directory to
the current directory and patterns can match both (pathless) file
names and file paths.

Given that it is convenient for 'make clean' to delete ignored files
(via the bin/clean script), there are conflicting desires: one the one
hand, take advantage of Git's richer semantics to specify ignored
files; on the other hand, not require git and a complete MLton git
repository to query ignored files (e.g., for a source release).

The compromise is introduce the bin/ls-ignore script to list the
ignored files of the current directory by constructing a 'find'
expression that matches names of ignored files.  The script supports a
reasonable subset of gitignore(5) semantics, but notably does not
support leading-"!" negated patterns and internal-"/" FNM_PATHNAME
patterns due to the complexity of replicating their semantics.
Rename each .ignore file to a .gitignore file.

After the SVN repository to Git repository conversion, a '.gitignore'
symlink was created for each '.ignore' file
(commit 9eecf8c70a40a2f6f0bc08a05efd9e8731ccabdf).  This was simple
and avoided the need to audit other uses of '.ignore' files.  With
commit 926517ffc4dbdaf16dfffbab09255abe46ab2302, the 'bin/clean' and
'bin/ls-ignore' scripts support a reasonable subset of gitignore(5)
semantics, so there is no need to maintain both .ingore files and
.gitignore symlinks.