* src/roff/troff/input.h: s/COMPATIBLE_SAVE/PUSH_GROFF_MODE/.

* src/roff/troff/input.h: s/COMPATIBLE_SAVE/PUSH_GROFF_MODE/.
s/COMPATIBLE_RESTORE/POP_GROFFCOMP_MODE/.
Update all users.
(PUSH_COMP_MODE): New internal character constant.
Update all users.

* src/roff/troff/input.cpp (get_copy): Handle PUSH_GROFF_MODE,
PUSH_COMP_MODE, and POP_GROFFCOMP_MODE.
(token::next, macro::append): Handle PUSH_COMP_MODE.
(decode_args): Add PUSH_GROFF_MODE or PUSH_COMP_MODE before and
POP_GROFFCOMP after each of the decoded arguments to make them
independent from the compatibility mode status.
(comp_mode): Add `COMP_ENABLE'.
(do_define_string, do_define_macro): Handle COMP_ENABLE.
(define_string, append_string, define_macro, define_indirect_macro,
append_macro, append_indirect_macro): Handle `compatible_flag'.
(chop_macro, substring_request, asciify): Handle PUSH_COMP_MODE.

* man/groff_diff.man, doc/groff.texinfo: Updated.