discover/grub2: improve handling of word & delimiter tokens

Currently, the delimiter token handling is a little fragile: we try to
ignore non-inter-word delimiters in the lexer with a selective set of
regexes on the possible delimiter characters.

This means we don't need to handle potential delimiters in every grammar
rule, but there are other situations (not regex-able) where we may see
delimters, and this will cause a parse error.

Instead of relying on the regex behaviour, we have an 'inter_word' flag,
which is set when we see the first word token, and cleared when we see
an end-of-line token. We only emit TOKEN_DELIM when this flag is set.

This means that we only get the delim tokens when they're required -
when we're looking for word separators (becuase WORD DELIM WORD is
distinct from WORD WORD - eg "linux /vmlinux" and "x$var").

We add a few new tests for the "menuentry" and "if" syntax, with
different delimiter configurations.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

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