[bug-recutils] Uni-char field names

[Jose E. Marchesi]

Hi guys.

I found an embarrasing small mess regarding field names composed by a
single character:

- The parser and generally librec was assuming that a field name is of
  the form [a-zA-Z%][a-zA-Z0-9_]* but...

- The selection expressions lexer used {LETTER}[a-zA-Z0-9_]+ and

- The user manual was claiming that field names are of the form
  [a-zA-Z%][a-zA-Z0-9_]+

So what I did (change pushed) was to change both the sex lexer and the
user manual so uni-char field names are both supported and documented.
I also added a test to make sure this will still work in the future.

Incidentally '%' is now a special field name... maybe at some point we
will find something interesting to do with it :)