- token.def: enable T_DOT_IDENT
- tunit.h: add S_DOT_IDENT and make it share attributes with S_DOT_FILE,
describe available section types, add tunit_strtab_section_enter(),
rearrange declarations
- tunit.c: implement tunit_strtab_section_enter()
- parse.c: generalize .file parser, use it also for .ident
- input.c: handle S_DOT_IDENT, lookup or create .comment strtab section,
append given string literal to that section
- add optional ->output() method to struct section
- make output_section() use ->output() if present
- make strtab_init() set up ->output
- change strtab_write() into a trivial wrapper for output_section()
