Most of this is adding const keywords, but setvar() in var.c had to be
changed somewhat more.

sh: Fix crash when undefining or redefining a currently executing function

Add a reference count to function definitions.
Memory may leak if a SIGINT arrives in interactive mode at exactly the wrong
time, this will be fixed later by changing SIGINT handling.

PR:		bin/137640
Approved by:	re (kib)

demanded by POSIX.
- A redirection error is only fatal (meaning the execution of a shell script is
  terminated) for special built-ins.  Previously it was fatal for all shell
  builtins, causing problems like the one reported in PR 88845.
- Variable assignments remain in effect for special built-ins.
- Option or operand errors are only fatal for special built-ins.
This change also makes errors from 'fc' non-fatal (I could not find any reasons
for this behaviour).

Somewhat independently from the above down-grade the error handling in the
shift built-in if the operand is bigger than $# from an error() call (which is
now fatal) to a return 1.  I'm not sure if this should be considered a POSIX
"operand error", however this change is needed for now as we trigger that error
while building libncurses.  Comparing with other shells, zsh does the same as
our sh before this change (write a diagnostic, return 1), bash behaves as our
sh after this commit (no diagnostic, return 1) and ksh93 and NetBSD's sh treat
it as a fatal error.

type of their argument, if it is a shell function, an alias, a builtin, etc.
-V is more verbose than -v.

PR:	77259, 84539

OK'ed by:	imp, core

- Removed dead declarations
- Made objects that should have been declared as static, static.

The changes use STATIC instead of static, following the existing
convention in the rest of the code.

Approved by:	schweikh (mentor)
MFC after:	2 weeks

o Old-style K&R declarations have been converted to new C89 style
o register has been removed
o prototype for main() has been removed (gcc3 makes it an error)
o int main(int argc, char *argv[]) is the preferred main definition.
o Attempt to not break style(9) conformance for declarations more than
  they already are.
o Change
	int
	foo() {
	...
  to
	int
	foo(void)
	{
	...

  PATH=... command

Noted by and fix works for Marcel Moolenaar <marcel@scc.nl>