all: avoid quoting file names when possible

Quote file names using the "shell-escape" or "shell-escape-always"
methods, which quote as appropriate for most shells,
and better support copy and paste of presented names.
The "always" variant is used when the file name is
embedded in an error message with surrounding spaces.

* cfg.mk (sc_error_shell_quotes): A new syntax check rule
to suggest quotef() where appropriate.
(sc_error_shell_always_quotes): Likewise for quoteaf().
* src/system.h (quotef): A new define to apply shell quoting
when needed.  I.E. when shell character or ':' is present.
(quoteaf): Likewise, but always quote.
* src/*.c: Use quotef() and quoteaf() rather than quote()
where appropriate.
* tests/: Adjust accordingly.

tests/df/total-unprocessed.sh

@@ -34,7 +34,7 @@ if test "$(df --output=fstype . | tail -n1)" != '-'; then
 fi
 
 cat <<\EOF > exp || framework_failure_
-df: '_does_not_exist_': No such file or directory
+df: _does_not_exist_: No such file or directory
 EOF
 
 # Ensure that df writes the error message also in the following case.