Wildcard grammar does not allow masks like "`*ar`"

[jwodder]

The ABNF for wildcard expressions currently requires that all occurrences of * or ? be preceded by a non-backslash character; as a result, the grammar disallows masks that begin with a wildcard that is not followed by another wildcard, such as "*ar" or "?oo". I suspect this is not the intention, and I doubt that many servers implement wildcards as written. A better grammar would be something like:

mask        =  *( nowildnoesc / wildone / wildmany / esc wildone / esc wildmany / esc esc )
wildone     =  %x3F
wildmany    =  %x2A
nowildnoesc =  %x01-29 / %x2B-3E / %x40-%5B / %5D-FF
                 ; any octet except NUL, "*", "?", "\" 
noesc       =  %x01-5B / %x5D-FF
                 ; any octet except NUL and "\"
esc         =  %5C

This also fixes the problem that an escape character can't be escaped.

[SadieCat]

Do any servers actually implement escapes? We don't.