Return a marker token at the end of any buffer

[Rangi42]

Removes the lexer hack mentioned in #778

[Rangi42]

An INCLUDE at the end of a file, without a newline after it, was previously relying on the newline-at-EOF hack. Without that, include has to be a line_directive, so the included file contents will not be tokenized until after seeing the T_EOB.

This fix will allow #716 to be mergable since it needs the EOF hack removed to allow [[ inline fragments ]] without newlines.

[Rangi42]

This also now allows a label in front of break, since (unlike most line_directives, REPT, IF, etc) there's no reason to prevent it.

[Rangi42]

The unterminated flag for reading REPT/FOR/MACRO contexts is also necessary because previously, the lack of a hacked T_NEWLINE was what stopped unterminated contexts from being evaluated. Now that T_EOB appears anyway, they need an explicit check (which also causes a more sensible error message for the nested-macrodef test case).