Shell completion for filenames from current directory, related docs.
* lisp/shell.el Shell completion now matches executable filenames from
the current buffer's directory, on systems in which this behaviour
is the default (windows-nt, ms-dos).
* src/callproc.c (Vexec_path): Document that exec-directory is in it.
* doc/emacs/mini.texi (Completion Options): Add a link to Shell Options.
* doc/emacs/misc.texi (Shell Mode): Move documentation of
shell-completion-fignore from Shell Mode to Shell Options.