watch: Nearly 30 bugs fixed, code optimized, no feature changes

Bugs fixed (commits in order) (details and non-bugfixes in commit msgs):

1. Missing WITH_WATCH8BIT separation.
2. my_getwc(): errors when char signed/unsigned, off-by-one in ungetc().
3. wcwidth() only if !WEOF, possible misaligned ptr dereference.
4. -w omitting blank line after truncated one, -w not triggering when wcwide char doesn't fit in last col [#285 (closed)].
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6. Blank line missing after a completely filled line [BUGS].
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8. -g+-q now coexist, -d no longer removes color, -c+-d don't mess up diffs, -q counter doesn't soar when resizing screen, malloc(sizeof(item_ptr)) -> malloc(sizeof(*item_ptr)). Manpage fixes (-x, BUGS).
9. No output to hijacked stdout/stderr after fork(), no leakage of fds or processes when EINTR, undone Debian #240989 (would let unprintables be output, may need review). Manpage fixes (-p, missing COLUMNS and LINES).
10. -qN becomes -q1 when N<1, don't reset -q counter when screen resizes. Header: a few text misalignment issues, consider wcwidth when WITH_WATCH8BIT, ENOMEM after asprintf(). Manpage fixes (non-existent BUGS).
11. Consume inadvertent input before exiting on -e.
12. -d no longer ignores combining characters [BUGS].
13. -d now considers full wcwidth of new character.
14. Orderly recovery from cmd taking longer than -n [BUGS].
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