Document fork, exec, wait and redirect2null Lua functions as deprecated

Rpm scriptlets should have no business dealing with this level of
detail in process control, rpm.execut() is much saner and safer
for scriptlet needs. We can't just remove because this is a public
API with known users (eg glibc in Fedora), but we can at least document
these as deprecated.

fork() and exec() are the main "problems" here, but wait() and
redirect2null() become meaningless once you take the first two away.

Fixes: rpm-software-management#2420

docs/manual/lua.md

@@ -292,7 +292,7 @@ f = f:reopen('r.gzdio')
 print(f:read())}
 ```
 
-#### redirect2null(fdno)
+#### redirect2null(fdno) (DEPRECATED)
 
 Redirect file descriptor fdno to /dev/null (rpm >= 4.16, in rpm 4.13-4.15
 this was known as posix.redirect2null())
@@ -307,6 +307,8 @@ elseif pid > 0 then
 end
 ```
 
+This function is deprecated, use `rpm.execute()` instead.
+
 #### undefine(name)
 
 Undefine a macro (rpm >= 4.14.0)
@@ -440,10 +442,10 @@ if not posix.chmod(f, 100) then
 end
 ```
 
-#### exec(path [, args...])
+#### exec(path [, args...]) (DEPRECATED)
 
 Execute a program. This may only be performed after posix.fork().
-For executing external commands it's recommended to use `rpm.execute()` instead.
+This function is deprecated, use `rpm.execute()` instead.
 
 #### files([path])
 
@@ -456,10 +458,10 @@ for f in posix.files('/') do
 end
 ```
 
-#### fork()
+#### fork() (DEPRECATED)
 
 Fork a new process. 
-For executing external commands it's recommended to use `rpm.execute()` instead.
+This function is deprecated, use `rpm.execute()` instead.
 
 ```
 pid = posix.fork()
@@ -724,7 +726,7 @@ posix.mkdir('aaa')
 posix.utime('aaa', 0, 0)
 ```
 
-#### wait([pid])
+#### wait([pid]) (DEPRECATED)
 
 Wait for a child process. If pid is specified wait for that particular child.
 
@@ -737,6 +739,8 @@ elseif pid > 0 then
 end
 ```
 
+This function is deprecated, use `rpm.execute()` instead.
+
 #### setenv(name, value [, overwrite])
 
 Change or add an environment variable. The optional overwrite is a boolean