bc: yyparse() loop regression

[mknos]

• A previous commit replaced a goto into yyparse() with a call to yyparse() but this introduced an infinite loop for a file with a syntax error
• yy_err_recover() returns bool; returning 1 instead of calling yyparse() again allows yyparse() to itself return and break out of "yyloop"
• The pattern of return(1) from yy_err_recover() was already established; this patch just makes it more consistent
• Regarding the removed yyclearin() call, that would be called from main()->clear_flags() before yyparse() would be called again
• test1: echo '1++' > A.bc && perl bc -y A.bc
• test2: echo '~' > B.bc && perl bc -y B.bc

%echo '1++' > A.bc && perl bc -y A.bc   # before patch
yydebug: state 1, reading 0 (end-of-file)
yydebug: state 1, reducing by rule 2 (stmt_list_exec : stmt_list_exec stmt_exec)
yydebug: after reduction, shifting from state 0 to state 1
yydebug: state 1, error recovery shifting to state 2
"A.bc", line 1: syntax error
yydebug: error recovery discarding state 2
yydebug: state 1, error recovery shifting to state 2
yydebug: error recovery discarding state 2
yydebug: state 1, error recovery shifting to state 2
yydebug: error recovery discarding state 2
yydebug: state 1, error recovery shifting to state 2
yydebug: error recovery discarding state 2
LOOP