expr: lint cleanup, and introducing main_exit

expr: lint cleanup, and introducing main_exit

This introduces a new macro main_exit, which is useful
for pacifying gcc -fsanitizer=lint and in some cases
means we can remove some ‘IF_LINT’ and ‘ifdef lint’ code.
* src/expr.c (main): Use main_exit, not return.
(docolon): Omit an IF_LINT that GCC no longer needs.
* src/system.h (main_exit): New macro.