Increase shcomp bytecode header version; doc updates

src/cmd/ksh93/include/version.h:
- Centrally define the 93u+m copyright (SH_RELEASE_CPYR) for adding
  to the original AT&T copyright in 'ksh --man' and 'shcomp --man'.
- Centrally define the binary header version number for bytecode
  generated by shcomp: SHCOMP_HDR_VERSION.
- Bump SHCOMP_HDR_VERSION from 3 to 4. Converting all the preset
  aliases to builtin commands has caused new bytecode to be
  incompatible with old ksh. (However, old bytecode runs fine on
  93u+m, because shcomp pre-expands the preset aliases.)

src/cmd/ksh93/sh/shcomp.c:
- Instead of keeping its own version date (not changed since 2003),
  use the same version string as ksh itself (SH_RELEASE).
- Use SH_RELEASE_CPYR for the extra 93u+m copyright string.
- Use SHCOMP_HDR_VERSION for the bytecode header.

src/cmd/ksh93/sh/parse.c: sh_parse():
- Use SHCOMP_HDR_VERSION for the bytecode version check.

src/cmd/ksh93/data/builtins.c: opt_ksh[]:
- Use SH_RELEASE_CPYR for the extra 93u+m copyright string.

src/cmd/ksh93/COMPATIBILITY:
- Mention that 93u+m shcomp bytecode won't run on older ksh.
- Document changes in printf %T (re: 9526b3fa).

src/cmd/ksh93/README:
- Mention that we run on UnixWare (with major regressions).
  https://github.com/ksh93/ksh/pull/159#issuecomment-764667929

src/cmd/ksh93/data/builtins.c

@@ -1486,7 +1486,7 @@ USAGE_LICENSE
 const char sh_optksh[] =
 "+[-1?\n@(#)$Id: sh (AT&T Research) "SH_RELEASE" $\n]"
 USAGE_LICENSE
-"[-copyright?(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK "]"
+"[-copyright?" SH_RELEASE_CPYR "]"
 "[+NAME?\b\f?\f\b - Shell, the standard command language interpreter]"
 "[+DESCRIPTION?\b\f?\f\b is a command language interpreter that "
 	"executes commands read from a command line string, the "