Handle string aliases for character tokens correctly.

Handle string aliases for character tokens correctly.
* src/symtab.c (symbol_user_token_number_set): If the token has an
alias, check and set its alias's user token number instead of its own,
which is set to indicate the alias.  Previously, every occurrence of
the character token in the grammar overwrote that alias indicator with
the character code.
* tests/input.at (String aliases for character tokens): New test.