Formeta: Escape leading and trailing whitespace

If names and values are not quoted then leading and trailing white space
must be escaped in order to make it part of the name/value. This commit
modifies the Formeta formatter to output the  extra escapes required.
Formeta: Only accept allowed escape sequences

The Formeta parser used to allow any character in an escape sequence.
Most characters were simply replaced with themselves. This behaviour
makes the introduction of new escape codes in the future very difficult.
Thus, this commit changes the parser behaviour so that only the
currently used escape sequences (nr",:{}\ and whitespace) are allowed.