Shell completion for filenames from current directory, related docs.

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.

Fixes: debbugs:15461