Purpose of smbrun, let's document it

[Andrew Tridgell]

> Last time I checked, the smbrun man page gave only a very sketchy
> explaination of its purpose.  It suggests that "some systems" require it.

smbrun is only used on systems that don't have execl(). Most unix-like
systems do have execl().

It is used to securely launch external programs, which mostly means
lpr, lpq and programs specified in preexec options.

> What I want to know is if it is OK to disable the use of smbrun.

if your OS has execl() then you can remove it. Does anyone want to
submit a patch to make smbrun not compile/install on systems with
execl() ?