At first glance, this function seems useful, but it actually increases
the amount of code required rather than decreasing it. Inline the
logic into the callers instead; most callers don't use the 'lexeme'
argument for anything and as a result considerable simplification is
possible.

Along the way, fix the header comment for the nearby function
lex_expect(), which mislabeled it as lex_accept().

Patch by me, reviewed by David Steele, Mark Dilger, and Andrew
Dunstan.

Discussion: http://postgr.es/m/CA+TgmoYfOXhd27MUDGioVh6QtpD0C1K-f6ObSA10AWiHBAL5bA@mail.gmail.com