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/include/version.h

@@ -21,6 +21,7 @@
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
 #define SH_RELEASE_DATE	"2021-01-20"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */
 /* Arithmetic $((.sh.version)) uses the last 10 chars, so the date must be at the end. */
@@ -33,3 +34,13 @@
 #    define SH_RELEASE	SH_RELEASE_FORK "/" SH_RELEASE_SVER "+dev " SH_RELEASE_DATE
 #  endif
 #endif
+
+/*
+ * For shcomp: the version number (0-255) for the binary bytecode header.
+ * Only increase very rarely, i.e.: if incompatible changes are made that
+ * cause bytecode from newer versions to fail on older versions of ksh.
+ *
+ * The version number was last increased in 2021 for ksh 93u+m because
+ * most of the predefined aliases were converted to builtin commands.
+ */
+#define SHCOMP_HDR_VERSION	4